M14 Forum banner
1 - 20 of 21 Posts

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #1 · (Edited)
Part I

Folks, I thought it might be a good idea to make one sticky about headspace that people can refer back to as needed, rather than having short posts strung all through the forum. So let’s start with what Headspace is and why it is important.

Having the correct headspace is a HUGE Safety Concern in all firearms and especially in center fire rifle cartridges because the pressures involved are much higher than other firearms. Both the M1 Garand and the M14 operate at 55,000 pounds of pressure at the breech. Nothing to sneeze at or mess around with and this is serious business.

MINIMUM HEADSPACE: This is the amount of room necessary to ensure the longest cartridge manufactured to correct specifications will fit and will have enough room to expand to seal the chamber. There has to be enough room for the cartridge case to expand so there isn’t too much pressure on the rifle. I personally fixed a DCM M1 Garand that had exceptionally short headspace. (Don’t know how it got out of the Arsenal that way, but they never finish reamed the chamber.) When fired, that rifle cracked stocks within two clips of ammo and the owner said it “kicked like a mule.” I’m sure it did. Fortunately, when he told me about it I got him to STOP shooting the rifle until I could correctly cut the minimum headspace. Had he continued to fire that rifle that way, the super high chamber pressures generated because the cartridge case didn’t have enough room to expand would have at least indented his receiver and bolt, besides continuing to damage stocks. It would also have caused the bolt and receiver to crack, chip or break had he continued to fire them and his rifle would have wound up as a pile of junk. You don’t want a chunk of metal from the receiver or bolt coming back into your eyes, even if you wear shooting glasses.

MAXIMUM HEADSPACE: This is the amount of room necessary so the shortest cartridge manufactured to correct specifications will fit and will have enough room to expand to seal the chamber without expanding so far that the cartridge case will expand until it ruptures or a piece of the cartridge case breaks off. When that happens, you are going to get a good deal of nasty and hot gas coming back into the action and towards your eyes. God Help you if you aren't wearing shooting glasses, as you could very likely suffer permanent eye damage. Much of the gas coming back normally blows the magazine base out the bottom of the magazine well and cracks and splinters the stock along the sides of the magazine well at least. That usually isn't life threatening, but you may wind up with some good sized chunks of stock in your supporting hand/arm. I've seen up close one real, G.I. M14 blow up and while it didn't come close to killing the Marine, it would have taken out his eyes had he not been wearing shooting glasses.

Military rifles often allow more of a minimum to maximum headspace length to take care of the variance of Mil Spec ammo and because the firearms will be fired in bad weather, will fill up with powder residue when fired a lot and/or where sand, mud and other “junk” can get into the chamber. The headspace is also more generous because military rifles are fired faster and with more rounds fired faster. That means the barrel will heat up and thus “close in” the headspace when the barrel gets hot. The heat expansion will the actually cause headspace to close up a bit when the barrel really heats up. So, there has to be a little more headspace room than in civilian rifles. EDITED TO ADD: Military spec. cartridge cases also have thicker brass to take the strain of larger headspace dimensions as well.

OK, so how is headspace measured? Well, that’s different for different types of cartridges, so let’s just stick to the M1 and M14 rifle because this thread is going to get long enough as it is. Grin. Headspace in these rifles is defined as the space or distance between the face of the bolt and to a point in the barrel chamber for the angled shoulder for the cartridge case. That point is called the datum point (or sometimes Datum line) in the chamber. Correct Headspace overall length and the datum point for these cartridges are specified in government specifications and/or The American Sporting Arms and Ammunition Manufacturers' Institute; hereafter mentioned by the common acronym of SAAMI and pronounced “sammy.” (A little more on this later.) The government arsenals had some very precise special adjustable gauges to check headspace, but even they usually used Headspace Gages that are precision ground to a precise length. Almost every civilian gunsmith or armorer uses individual Headspace Gages to inspect or chamber a rifle. So let’s go over the most common ones next.

The “ GO “ Gage: This is the shortest Headspace Gage and is used to check and ensure the chamber has the MINIMUM headspace necessary for reliable and safe operation for even the longest cartridge case that is still inside manufacturing specifications.

The “ NO GO “ Gage: This is the most widely MISUNDERSTOOD Headspace Gage and causes the most confusion to a whole lot of folks. I can’t tell you how many times I’ve heard or seen incorrect things said or written about this Gage and sometimes by people who SHOULD know better. This Gage is used by Arsenals or gunsmiths/armorers when they cut the Headspace in the chamber of a rifle along with the GO Gage. You have to cut the Headspace deep enough so the GO Gage will fit without showing additional friction on the bolt. That gives you the Minimum Required Headspace. You then use the NO GO Gage to ensure you don’t cut too much Headspace when you first chamber a rifle. Headspace may increase as much as one or two thousandths of an inch ( .001” to .002” ) during the entire life of the barrel until you shoot the lands out. SO you have to have a NO GO Gage to show you where to stop cutting and ensure there will not be too much Headspace throughout the entire life of the barrel. (IOW, when you stop cutting the headspace - there will still be a few thousandths of an inch before you hit the Maximum Headspace length.) The NO GO Gage is NOT the maximum gage and it is NOT the Gage that tells you the Headspace is too much or too deep. If you don’t cut Headspace, you really don’t even need this gage to check for safe Minimum and Maximum Headspace.

The “ Field Reject “ Gage: OK, THIS Headspace gage is the gage you use to check for Maximum Safe Headspace even when you get the shortest cartridge made to specifications. If the bolt closes on this gage without additional friction than before you had the Field Reject Gage in the chamber, then the barrel is UNSAFE and no one should fire the rifle until the barrel is pulled and a new one is installed. If one wishes to have the two necessary Gages to check Headspace, you need this Field Reject Gage and the GO Gage for any individual caliber.

There are going to be additional posts containing additional information. So please stay tuned. Grin.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #2 · (Edited)
Part II

OK, remember from the above post that Headspace Dimensions can be taken from both the U.S. Government Arsenals and/or SAAMI specifications? Before I go further I want to add that the dimensions might APPEAR to be different between them when you only look at the listed length dimensions. This is really important and caused quite a ruckus a few years ago when SAinc. started writing down the headspace on a tag they attached to the M1 Garand rifles they built with commercial receivers. The Headspace readings they wrote down were hugely different from the normal readings we see from Government Gages or from Gages made to Government specs. That FREAKED OUT a whole bunch of folks because they thought the barrels had been improperly cut for Headspace. When I first ran across this, I used my Government Gages that had been calibrated and found the SAinc. headspace was fine – as I figured it would be. SAinc. used gages that had the SAAMI spec reading on them and that’s why the readings were different.

SAAMI gages in .30-06 have a different length reading because they are measured to a different Datum line on the angled shoulder of the gage. The angle is different than G.I. gages, BUT that doesn’t mean a problem, it merely means the Datum Point is in a different area on the gage. Since chamber reamers are also standardized, the chambers are going to be cut close to the same no matter who makes the reamer. Not everyone has run across this or may have even heard about it, so I thought it was important to mention this as it may be something someone might run into on a M1 Garand or even a M14. I’m not sure if SAAMI length readings are different for .308/7.62mm and never saw them listed differently than how the government gages were listed, but if you run across this, just check them with regular gages and know the rifles will be fine when they pass headspace inspection with regular Gages.

Now it’s time to switch to another widely held misconception or myth about headspace on both M14’s and M1 Garands that are chambered in .308 or 7.62mm. These two calibers are pretty close to being the same with the main difference being the angle of the shoulder of the cartridge case, BUT they are NOT the same caliber. 7.62mm Nato ammo also has a slightly longer throat dimension than .308 Win.

Headspace lengths for caliber .308 Winchester

1.630" GO
1.634" NO GO

The G.I. Headspace Gage lengths for 7.62mm

Go Gage...................................... 1.6355"
NO Go Gage................................ 1.6375"
Rebuild Maximum Gage.............. 1.6415"
Field Reject Gage....................... 1.6455"

NOTE: The information on Government Headspace Gage lengths was taken directly from the 3-5 Tech Manual and the Rebuild Standards manual published by Rock Island Arsenal.

Also, I would like to explain that "Rebuild Maximum Gage" reading of 1.6415". That is MY descriptive term for it because the Rock Island Arsenal Rebuild standards just list it as one of the three gages (by length) to be used for rebuild of real, G.I. M14's. Remember that Headspace should only grow at most one to two thousandths of an inch during the entire life of a barrel? If it goes much beyond that, you have a REAL problem with either the bolt or the receiver, or both. So, when they rebuilt rifles to 5th Echelon - Depot Repair Standards, they used 1.6415" as the maximum to ENSURE the barrel would not have a headspace problem during the entire life of a barrel. That “Rebuild Maximum Gage” ensures there is at least THREE thousandths of an inch less than the Field Reject of 1.6445", so there would almost never be a problem with Maximum Safe Headspace while the barrel was still good.

When we chambered (cut the Headspace) on real, G.I. NM M14 barrels for competition shooting in the Marine Corps, we used .308 Headspace Gages and chamber reamers. We used both NM G.I. Government “White Box” 7.62mm ammo and a proprietary Federal “NM type” .308 loaded ammo that has come to be called “Federal Gold Medal Match” in recent years. Everyone who doesn’t know should understand that NM ammo is loaded about 200 FPS LOWER and doesn’t develop quite as much chamber pressure as regular 7.62mm Nato ammo. Some NM Armorers in the Marine Corps (as well as some of the other Armed Forces NM Armorers) didn’t understand back in the early 1970’s that you can’t cut the chamber dimensions to a Minimum .308 Win. Headspace length and have both types of ammo perform properly in a Gas Operated Rifle. You can often, if not usually get away with it in a Bolt Action rifle, but NOT a “Gas Gun” as we called the M14. The reason for that is when you reload for a Bolt Gun, you only have to resize the neck. The expanded fired case will still chamber well in a Bolt Gun because of the mechanical advantage of the bolt action RAMMING the cartridge in the chamber. In a Gas Gun when you reload, you HAVE to resize the whole cartridge case or it won’t feed properly and will often get stuck in some chambers before the cartridge is fully seated. So in the real world, what did we find you have to do to chamber an M14 with NM 7.62mm ammo?

At first, we chambered NM M14’s pretty much the same way that NM M1903 rifles and other NM Bolt Action rifles had been chambered for decades. We chambered them as close to the minimum .308 Headspace length of 1.630” as possible. We also used Elliot Headspace Reamers that were absolutely the Ultimate Quality reamers in the 60’s and 70’s. These reamers were on the “tight side” of manufacturing tolerances. (Today, Clymer’s .308 Match Reamer gives a similarly tight NM chamber.) The Lake City, “White Box,” NM 7.62mm ammo in both 168 grain and 173 grain bullet loadings would function OK in most rifles, but not in all of them and not as accurately as we thought they should have with so tight of a chamber. We found that the rifles performed better and more accurately when chambered to between 1.631” and 1.633” when using NM 7.62mm ammo. Why was that? Well, look up above at the minimum chamber length for a 7.62mm GO Gage. That reading is 1.6335” for Nato Spec Ammo. We actually found it was better to ream Headspace a bit deeper before the answer hit us in a “No Scheit Sherlock” moment. What we did by lengthening the Headspace was to get it close to or right at the minimum 7.62mm chamber length even when using lower pressure NM ammo. So in the real world, it also shows up there is a difference between .308 and 7.62mm chambers. Well, that’s fine for NM or target grade barrels, but what about standard chambered rifle barrels?

Standard 7.62mm Nato ammo and ESPECIALLY foreign surplus ammo may or may not be held to true Nato specifications. One thing for sure is that standard 7.62mm ammo manufacturing specifications allows for a slightly longer cartridge case (and slightly longer neck) than .308 Winchester. Some Nato spec ammo like the really good Portugese 7.62mm Nato ammo is also a bit on the “hot” side of the 7.62mm range and that means slightly higher chamber pressures from the “Get Go.” So what does that mean in “the real world?”

If after reading all these posts on headspace you only remember two things, it is my most fervent hope you will remember these two things:

1. Do NOT use standard Nato Spec surplus ammo in a rifle chambered with a NM chamber and set up at the minimum headspace. Even if the rifle functions with no apparent problems, the chamber pressure of each round fired is significantly more than in a proper Nato Spec chamber. What you are doing is subjecting your bolt and receiver to more abuse and it will AT LEAST cause your bolt and receiver to wear out faster. If the cartridge case brass alloy of some of the questionable foreign surplus is used, you are also asking for cartridge case ruptures and gas and pieces of brass coming back into your face and possibly into your eyes.

2. If you are shooting 7.62mm ammo in a rifle chambered for .308 Winchester and the ammo does not feed and function properly and/or if the ammo has a greater “kick” or much louder noise than usual; PLEASE stop firing that ammo in your rifle. Now, this sounds like it should be a common sense reaction, but I have seen and heard of WAY too many people who continue to fire ammo that does these things. At a local range about 20 years ago, I was next to a guy who kept shooting some junk 7.62mm ammo is his M1A and about every 7th or 8th round wouldn’t chamber or he had trouble extracting the cartridge case. I warned him about the possible ramifications of firing that ammo, but he ignored the warnings. OK, I got up and moved all my stuff to the shooting bench the furthest away from him as I could and waited to shoot when he wasn’t shooting. He asked me why I moved and I told him I didn’t want to be close to that rifle when the cartridge case ruptured or his rifle broke and pieces of metal came flying off the rifle. He again ignored me and went back to shooting the rifle because, “I didn’t know what I was talking about.” Yep, he eventually ruined his bolt and receiver from firing that junk 7.62mm ammo, though fortunately in his case no one got hurt. God seems to sometimes protect even the worst idiots from their own bad actions at times.

OK, so CAN a rifle be chambered so that both .308 Winchester and GOOD 7.62mm Nato Ammo be fired in it without problems? The good news is yes it can. Most factories who make or fully assemble their rifles chamber them so you CAN shoot both, but it is not absolutely certain in every rifle and with every type of 7.62mm Nato Ammo. Please see Number 2. above.

Now, the GREAT thing is that if a rifle is chambered so that the bolt correctly closes on a 7.62mm GO gage at 1.6355" and does not close on either a 7.62mm NO GO gage of 1.6375" or the .308 Field Reject of 1.638" - then there are absolutely no worries about shooting either .308 nor 7.62mm in the rifle. This is how I chamber Garands and semi auto M14's for folks who want to shoot both .308 and 7.62mm surplus. (I chamber them so the bolt won't close around 1.635" to 1.636".)

If you use .308 Winchester Headspace gages, exclusively, you can still chamber the rifle correctly for 7.62mm as well. You want the bolt to just close without friction on the NO GO gage. That will ensure you have the minimum for 7.62mm and not too much headspace for the .308 at 1.638".

OK, that’s enough for this post. Stay tuned as more is coming folks. Grin.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #4 ·
A huge part of the reason I write these posts is so the information can be passed along to "The Next Generation" of gunsmiths/armorers. When I began NM conditioning these rifles, I was just BARELY 20 years old by a month, even though I had been a Sergeant of Marines for a few months by that time. When I graduated the Marine Corps MOS 2112 Rifle Team Equipment Repair course, I was just barely 21.

Sure there will be those who can work on these rifles in your Grandson's time. There won't be a huge number of them, but there will still be some of them. How long has the M1903 been around and people still work on them? I still fix guns that were made at least 200 if not 300 years before I was born. There will be gunsmiths and armorers working on them 200 or more years after I report to Marine Barracks, Gates of Heaven for guard duty - if I make it there. Grin.

Thank you for the kind words.

P.S. Oh, there isn't much room in my small shop for anyone else. My three Collie girls already crowd the bench space when I'm working. Grin.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #6 ·
Thank you, Gus, that explains why my Gunsmith insisted on my getting a Field Gauge as opposed to the other two varieties to keep an eye on Nancy Jean's headspace.

The problem, so far, has been finding a Field Gauge in 7.62x51 NATO. I HAVE found them for .308, but the other variety hasn't turned up anywhere.

Two sources for a G.I. Field Reject Headspace gage come to mind, though they might not have them currently in stock, they sometimes run across them. Those sources are 1. Our own Bill Ricca, who adds so much of his expertise to the forum and 2. Tony Pucci at Orion 7.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #7 · (Edited)
Part III

Sooner or later this discussion leads to which Headspace Gages are “best,” so let’s take that next.

I mostly use Clymer Headspace Gages as I’ve had them calibrated by the Marine Corps Calibration Laboratory at Albany, GA and they ALWAYS came though calibration inspection with flying colors. I have also used and can recommend Manson, JGS, and Pacific gages. That’s it folks, there are no others I recommend.

What is SUPER important about Headspace Gages is that you stick to the EXACT SAME BRAND for each caliber. That means you buy the GO Gage, NO GO Gage and Field Reject Gage from the same company. If you don’t do that, you are going to get different or even wrong readings due to the different ways the makers manufacture the gages and their own tolerances - if you mix manufacturers in your gages.

If you aren’t going to do any chamber reaming/cutting, you only need the GO and Field Reject Gage for any caliber to make sure you have safe headspace. Some folks buy the NO GO gage just to give them an idea of how tight or loose the headspace is and there is nothing wrong with that, just understand you don’t need the NO GO Gage to tell you if the headspace is safe.

OK, someone out there has probably noticed I have not mentioned Forster Headspace Gages and is wondering why. Well, there is another story involving Headspace Gages, so I may as well relate it right now.

When I was transferred to Edson Range aboard Camp Pendleton, CA; among the RTE rifles there were NM bolt action Model 70 rifles in .30-06. They were one of the last Post and Station Teams that still had those rifles and they had kept them because they used them in the California Long Range Championships held at Marine Corps Base 29 Palms. Well, any rifle the Marine Corps uses HAS to be periodically given a technical inspection before it can be fired and that includes the use of Headspace Gages to inspect them.

One day my Gunner walked into my RTE Armory and was looking around. He liked the Model 70’s, but then he got a quizzical look on his face and asked me if I had a set of Headspace Gages to check those rifles. Well, I told him I had my own personal .30-06 Gages and used them. Then he told me he appreciated that, but when was the last time they were calibrated? OOOooops, well mine had never been calilbrated after they left the manufacturer. So, it was decided we should order two sets of .30-06 GO, NO GO and Field Reject gages and keep them as Team/Marine Corps property. We had to have two sets so one set would be in calibration and be used when we sent the other back for annual recalibration and vice versa. Note: In case anyone is wondering why we didn't order G.I. Headspace Gages for .30-06, they were no longer available through the Federal Supply System.

So, we dug out Brownell’s catalog and ordered two sets of Forster Gages because they were the least expensive. (Many folks start out buying Forster’s for that reason.) Then it took about a dozen phone calls till I found a Calibration Tech at Albany who assured me they could calibrate the gages even though they were not the G.I. pattern. GREAT ! ! So we packed up both sets of gages and sent them off. They told me they would have to work them into calibration cycle when they had slow time from other Infantry Weapons gages. No problem there. About three weeks later, I called to see how it was going and got some bad news. ALL of the six Forster gages FAILED calibration by a good deal. Awwwww………..crapola! That normally meant we would have to get another 6 gages. Fortunately, the Calibration Tech then told me they were all too long and they could most likely properly grind them down to size. Well, that worked on 5 of the six gages, but the sixth Gage was a Field Reject gage and it was just too short to regrind. So, we had to order another gage and that one was long enough, though too long as well and had to be reground.

I also got the Calibration Laboratory to calibrate a set of .308 Winchester Headspace gages that had been issued to me, but those were Clymer Gages. They passed calibration with flying colors. In the late 1980’s, Clymer Gages cost two or three bucks more each than Forster gages. A few bucks difference, BUT the Clymer gages passed calibration when the Forster gages failed. Sadly, a case of poor economy by trying to save a few bucks with the Forster gages.

When I returned to the RTE Shop at Quantico, they had not had their gages calibration inspected for a while, so I called my Calibration Tech at Albany and got half of our gages sent in. Once again the Clymer and this time the JGS gages we had passed and the Forster gages FAILED. Only about half our Forster gages could be reground so they would pass calibration. After that, I made sure we at the RTE Shop and when I personally bought my own gages, that we never bought Forster Gages again. I still have one .308 GO Forster gage, but it is almost as long as a NO GO gage. Since I know that, at times it comes in handy, though I DON'T use that gage to check Minimum Safe Headspace. I threw away my personal Forster .30-06 gages and other .308 gages because as a Armorer, I can’t use gages that will not meet or hold calibration due to liability concerns. That’s why I don’t recommend Forster gages.

One last thing on Headspace Gage brands and that is you don’t have to worry about using a chamber reamer from a different maker than your Headspace Gages. Since you are going to cut the chamber, it doesn’t make a difference who makes the reamer as you cut the chamber to size using your Headspace Gages. IOW, you can use a set of Clymer Headspace Gages and a JGS reamer and you will be just fine.

Yes folks, there is STILL more to come, so stay tuned.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #8 ·
Part IV

OK, so how do you properly check Headspace in your rifle? The first thing you MUST do is ensure you have a clean and dry chamber that doesn’t have oil, grease or powder residue in it. Same thing with the face of the bolt. To get the chamber clean, I strongly suggest you use the ratchet style chamber brush and use the correct one for the caliber rifle you have. IOW, use the shorter G.I. one in .308/7.62mm as in the M14 and the longer G.I. one for the Garand. The receiver bolt lug area must also be clean and free of foreign matter. If you don’t have these parts clean and dry, you are GOING to get a false reading on your chamber headspace.

The next thing you should do is strip the bolt so the ejector and extractor doesn’t hang up on your gage, or against the inside of the barrel shoulder. Yeah, I know some folks say this isn’t necessary if your gage is relieved for them, but after checking headspace on thousands and thousands of Garands, M14’s and numerous other firearms over the years – I’m here to tell you that you will occasionally get a false reading if you don’t strip the parts. The extractor is the usual culprit for that, but even the ejector can hit or rub on a gage that has supposedly been relieved for it. This is even more true and more common with civilian made semi auto M14 receivers as the receiver bolt lug area on many of them is not nearly as precisely milled out as G.I. M14 receivers.

The first step is to put the bare bolt in the receiver and ensure the bottom flat of the right bolt lug goes all the way down onto the flat surface of the receiver. If it doesn’t on an M14, your bolt roller is probably hitting the receiver and the receiver has to be cleared for it. Move the bolt up and down a few times to get the “feel” of how much friction there is when the bolt comes all the way down on an empty chamber. That is super important as “feel” in headspace checking is very important. Some bolts go up and down with no noticeable friction, while you feel varying degrees with others.

After you have a good feel for the friction on the bare bolt going all the way forward and down, then pull out your GO gage. Pull the bolt back and push the gage into the chamber by hand. Don’t force it because if you have to force a gage, there is something wrong or you are doing something wrong. Lightly run the bolt forward and see if the right bolt lug goes all the way down flat – just like it did without the gage in place. If it does and you feel NO additional friction whatsoever on the bolt than without the gage, it passes the GO Gage test and has the minimum required headspace. If the bolt does not go all the way down flat OR if you feel ANY additional friction/resistance when the Go gage goes down, then it does NOT pass the Go Gage inspection and is often referred to as having “short” headspace. The fix for that is to 1. Change bolts, or 2. Lap the bolt lugs just a bit or 3. Cut the Headspace a little deeper. There are good and bad points about all three of these, so we should go into that next for each one.

1. Change bolts - This is fairly easy to do with a Garand as there were so many bolts made and they all fit correctly. They also came in a variety of lengths of distance from the rear of the bolt lugs to the face of the bolt, so you can often find a bolt that is a bit looser or tighter, depending on what you need. This is not nearly as easy to do with a commercial M14 receiver as the bolt lug area is not made nearly as precisely as the G.I. spec. receivers. Many bolts had to be lapped to get good even bolt lug contact with commercial receivers. Putting an unlapped bolt in the rifle can give you a MUCH different headspace AND the bolt lug contact may not be right. ALSO, G.I. M14 bolts varied very, Very little in lengths over the course of manufacture so you don’t see as much difference with them as you do with Garand bolts. Sometimes a worn G.I. bolt will give you just enough less length and that might be enough less that you wouldn’t have to worry about doing anything else to a chamber that is just a bit too tight.

2. Lap the bolt lugs a bit more. – This is especially useful if you didn’t originally cut the chamber in the rifle or if the chamber is chrome lined. If you didn’t cut the chamber, your headspace reamer may not be quite the same size up front as the reamer that cut the original chamber. If your reamer just happens to be a bit smaller, cutting a little more would leave a shoulder in the chamber you don’t want. (Then you have to polish the chamber or cut it even a bit deeper.) If the chamber is chrome lined and you don’t have a tungsten carbide chamber reamer, all you can do is lap the bolt lugs more. You do have to be watchful of how much material was taken off the bolt lugs to lap them in to begin with as there is only about .015" of surface hardness on the bolt lugs and you don't want to lap/cut through the surface hardness. Usually this is best done by a qualified gunsmith or armorer.

3. Cut the headspace a bit deeper with a reamer. – This can only be done if the chamber is not chrome lined or if you have a tungsten carbide chamber reamer. (The last are REALLY expensive, so many armorers don’t have them.) When you cut the chamber a bit deeper, you have to ensure you cut enough so as not to leave an extra shoulder in the chamber because of the difference between your reamer and the one that chambered the rifle. Again, this is usually best done by a qualified gunsmith or armorer.

If the rifle passed with the GO Gage, now it is time to try the Field Reject Gage. Take the GO Gage out and put the Field Reject Gage in and test it like you did with the GO Gage. This time, you are looking to see that the bottom of the right bolt lug does NOT come all the way down to the flat area on the receiver. If that is what happens, your rifle is good. If the bolt comes all the way down, BUT there is additional friction on the bolt, the rifle is also good – though the headspace is rather long and you won’t get as many reloads out of the rifle because the brass is stretched more. If the bolt closes all the way down with no additional friction, then the rifle fails the Field Reject test and you must either find a bolt that is a bit longer or rebarrel the rifle.

There is still a little more, so stay tuned.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #9 ·
Part V

I’ve been thinking about how to best describe getting the “feel” for using headspace gages and it is a lot more difficult to get from something you read rather from something you can actually feel.

When I was the Instructor for OJT’s (On the Job Trainee’s) at the RTE Shop, I came up with using a barreled Garand action and a few bolts that I knew would give a different “feel” when headspace gages were used. That way, the OJT's could actually feel what I was talking about. Garand bolts were manufactured to different tolerances over the years so it was a LOT easier to find bolts that would give a different feel. M14 bolts were manufactured to much closer tolerances and that’s why you don’t see nearly as much difference with them. We actually stored those bolts in a separate plastic bag and had them listed as “Special Tools, Instructional Aids” on our inventory, so even FSMAO inspectors wouldn’t give us scheit about them. They were known as “Fisher’s Bag of Bolts” to the supply guys. Grin. I got some raised eyebrows about it at first from the FSMAO inspectors, but after explaining the use – they actually agreed. The heads of Garand Bolts and M14 bolts are almost identical and are identical when you just check headspace without the parts in the bolts. OK, so let’s get on with the “feel” of how the bolt closes on Headspace gages.

Gunsmiths and NM Armorers are not machinists, though we do some machine work. I have the highest respect for real machinists as they can do a lot that I can’t do, just because there is a lot to gunsmithing that doesn’t involve standard machine practice. So for the real machinists out there, please bear in mind with me as I play a little fast and loose with some machinist terms.

There are three basic types of “fit” when metal parts fit together. They are:
1. A clearance Fit (loose)
2. An interference Fit (tight)
3. A transitional Fit (between loose and tight)

These types of fit also have different classes of fit in each of the three types of fit. I don’t want to have most folks eye’s roll into their heads describing all of them, so I will pretty much stick to the three types of fit and ask real machinists to excuse me for being less descriptive for brevity. If anyone wants to read more on this subject, here’s a good short version link:


What we want when we use the GO Gage is a Clearance Fit so that you can not feel any more friction or resistance as you close the bolt on the gage. That and ONLY that is the proper way to ensure you have minimum headspace.

If you have a Transitional Fit, then you can feel some additional friction or resistance as the bolt closes on the Gage. This is NOT acceptable when you close the bolt on a GO Gage and it doesn’t pass the GO Gage inspection. What is going on is that while you can close the bolt all the way down, the metal in the bolt and chamber actually are bending ever so slightly so the bolt can close on the gage. This is because the barrel is softer than the headspace gage and while the bolt has a hard surface, it has a soft inner core so it doesn’t shatter when the cartridge is fired. There is a little “give” even from the bolt for that reason.

If you have an Interference Fit, you sometimes can still force the bolt closed down over the gage. However, you usually then have to use a screwdriver of some kind of small pry bar device to get the bolt to open. This comes from the metal in the barrel and the bolt having to bend or “give” even more so the bolt can close on the Go Gage. This is even less acceptable with a GO Gage than a Transitional Fit.

Now, I have heard even some people who SHOULD know better say that as the bolt closes on the GO Gage (no matter if there is a transitional or interference fit) then Headspace is still good. They can’t be more mistaken because in both cases metal has to be slightly bent or “give” for the bolt to close on the gage.

As far as the Field Reject Gage goes, the best is the bolt doesn’t go all the way down to the flat shoulder on the receiver. However, as long as you have either a Transitional Fit (additional friction) or BETTER STILL an Interference Fit (really tight and you have to pry the bolt open) then Headspace is still safe.

Personally, I would not advise shooting a rifle where there is only slight additional friction or resistance when the bolt closes on the Field Reject Gage. You are very close to the reject point and it is better to get a new bolt or barrel and get that headspace tightened up. Cartridge cases really stretch when fired in those chambers and a weak case or one that has been reloaded a few times is likely to rupture. However, you can still shoot a rifle where there is an Interference Fit between the bolt closing on the Field Reject Gage. I would PLAN on getting a new barrel or bolt in the very near future though, if you are going to shoot that rifle a lot. Many original M 1917 .30-06 caliber rifles actually were chambered somewhat close to that and that’s the reason why today so many of them fail headspace.

OK, I think that’s about it for now, but might come up with more in the future.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #12 · (Edited)
Thanks for the great post. I'm about to do my first complete AR15 build and almost have all the parts for my m14 build (I'll have an armorer do that one). I've been reading and re-reading about headspace in preparation. Your thread definitely cleared up a few things, especially regarding the No Go gage. I guess I'll have to call Orion 7 and have them add some USGI gages to that order I just placed!
Well, that leads me to add something else. Finding original G.I. Headspace Gages for the 7.62mm is durn near impossible - except for the Field Reject Gage because that gage (and only that gage) came with all the 3rd Echelon Gage kits. The Arsenals were the only ones who had G.I. GO , NO GO and what I have called the Rebuild Maxiumum Gage. The same is true for the M16 as there was only a Field Reject Gage issued. There were so few Arsenal Gages, I doubt most folks can find them.

Now, if you want one each of those gages, that's fine. Just know that if you want the GO (and/or the NO GO) gages for both, then you are most likely going to have to get commercial gages.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #14 ·

Sorry I missed your post until just now.

Reading over what you wrote, I think you will be OK for most ammo. If you run into some surplus ammo that doesn't feed or extract well, then I wouldn't use it in that rifle.

I'm sorry that's the best I can say without examining the rifle personally.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #19 ·
Has anyone had experience with gauges from Pacific Tool and Die (PTG)? They sell them at Midway.

Thanks, Gus, for your outstanding explanation and experiences.
They make really good stuff. We use some of their headspace and other gages in the Armalite Police Armorers Course for "that other gun." Grin. One thing, though, it is always best to get all three of your headspace gages from the same manufacturer and not mix the gages.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #21 ·

You didn't mention it, but I assume you have a tapered shoulder on the snap caps that corresponds to the shoulder on the cartridge case just before the neck? Even if not, you can still run into problems with length if you have no shoulder on the snap cap. Here's why.

The Datum line where the forward point of headspace is measured is different between Military and SAAMI chambers and headspace gages even though there is no difference between the .30-06 in Military or Commercial. I stole this link from another post to show you what we mean by the different Datum Lines. In the "Long Chamber" drawings of the link, this illustrates the difference between 7.62mm and .308. The lines going through the shoulder areas show different Datum Points (areas along the shoulder) where headspace is checked.


Now, I only VERY rarely have used SAAMI spec. gages in .30-06 as I have a complete set of in spec Military Gages. BUT SAAMI gages read much longer than Military gages for each measurement of GO, NO GO and Field Reject. This means the SAAMI gages have a different angle that goes up against the wall of the angled shoulder in the barrel. That also means the angle of the shoulder of the SAAMI gages are less than the angle on the Military Gages - otherwise the SAAMI gages would not go in as deep and read as long as they do.

I'm not sure if this is clear, but if not, please ask more questions.

We REALLY see the difference in the shoulder angles of Headspace gages in .308 gages vs Military Gages in 7.62mm with the SAAMI gages also having less of an angle.

What I'm pretty sure you are running into is the shoulder on your snap caps don't have enough angle (meaning the angle isn't tapered enough) and can't slip far enough into the chamber on .308 (or .30-06) in Military Chambers. If you reduce the angle of the shoulder on your snap caps and maybe extend the shoulder back towards the rear of the cartridge case, the snap caps should fit fine in both Military and SAAMI spec chambers. IOW, it would be best to fit them to the SAAMI spec and then a .308 snap cap will work well in a 7.62mm chamber as well.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #26 ·

I may not have gotten my point across correctly. Military Chambers are allowed to be longer than SAAMI chambers and that's why I recommended you size the snap caps to SAAMI specs for both .308 win and .30-06. The SAAMI specs call for shorter chamber lengths, so if you make the snap caps so they go in easily in SAAMI chambers, there should be no problems with the length to fit military chambers.
This includes a sharper taper for the shoulders of the gages.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #29 ·
-It's good to go for .308 spec ammo, being close to minimum headspace.

That's correct.

-It'd be too tight to safely use 7.62 spec ammo, being .002" or so short.

This may get a bit confusing, so please ask questions later if you don't fully understand.

The White Box NM 7.62mm ammunition was used in these kinds of chambers for NM shooting. This ammunition ran 200 - 300 FPS slower than regular 7.62mm ball ammo and thus the pressures weren't as high.

The 7.62mm ball or other 7.62mm ammo the U.S. government accepted could be safely fired in this chamber as the cartridge cases were held to tighter specs. and were stronger than normal .308 brass. BUT there is a Caveat to that. The U.S. government did not surplus ammo just because they had too much of it. They surplused it when it wasn't made correctly, had the wrong pressure curve, bad primers, etc., etc., etc. The average person has no way of knowing if the U.S. government surplused ammo due to rejection it didn't meet spec. or safety issues. Personally, I would probably fire any U.S. made 7.62mm ammo (except pressure test loads of course) and keep an eye on the brass or anything unusual. This to check if the ammo was surplused due to rejection or safety issues. If the U.S. government didn't want it, I don't want it in my rifle.

GOOD Nato spec 7.62mm also falls into the category I would at least try it to see if there are any problems on why it was surplused. But remember, countries also surplus ammo that is too old, wasn't stored correctly and you may have problems with the rounds going off. That's not really a safety issue, but could show up as some to many failure to fires.

Some 7.62mm ammo that is sold as "surplus" is not Nato spec. I would NOT fire that in this rifle's chamber as the cartridge cases could be too long or the pressure curve wrong or a number of other things.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #31 ·
Though a bit hot, I've fired the Portugese ammo in the old heavy OD plastic ammo packs for years. Not sure if I would use a lot of it, but the cases always seemed to be within tolerances so they would headspace properly.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #36 ·
...I am a bit confused, but I am easily confused...please bear with me...

...the tag that came with my new SAI M1A NM says it has been head spaced to GI specs and requires the use of GI mil spec or commercial equivalent...

...then I note your advice to not use NATO spec ammo in NM chamebers...it seemed that commercial 308 ammo would be ok though...???...

... the few rounds of ammo fired indeed show some mild symtoms of excess chamber pressure and are marketed as mil spec equivalent but are marked 308...

...how aboot the M118 load...???...is that correct for a 1.631 NM chamber...???...I am guessing that the M118LR load is not for use in this rifle which is a pisser as that is the type of ammo I had envisioned useing in this rifle...

...I guess I should have done this research before I ordered 440 round of remanufactored LC NATO spec 7.62x51 ammo...

...at this point I am feeling tightly headspaced...
NM chambers are on the tight side of .308 spec, but they are still good for most GI or GI equiivalent ammo. Where problems can/will come in is if you use NON Nato Spec 7.62mm ammo.

As long as that ammo is remanufactured correctly, you should be OK.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #41 · (Edited)
Well, the information that Ole Silver and JWB provided on more recent readings of 1.636" on current .308 Win. NO GO Headspace Gages is very interesting. EBRbuilder has also mentioned that they used a 1.640" Rebuild Maximum Gage at Rock Island Arsenal when they built the EBR's rather than the older 7.62mm Rebuild Maximum Gage that read 1.6415" long.

So it looks like it is a time for an update.

The common, if not standard that .308 Win. Headspace NO GO Gages were made to in the 70's through at least the early 90's was 1.634". So I don't confuse folks, here is a line up of headspace gage readings:

.308 Win.

1.630" GO
1.634" NO GO - I guess we can now say this was the "Old Common Length."
1.636" NO GO - I guess we can now say this is the "New" Common Length?
1.638" FIELD REJECT - I guess we can now say this is the Common Length or even "Old Common Length."
1.640" FIELD REJECT - New Length SOME manufacturers are using AND Rock Island Arsenal used as the "Rebuild Maximum Gage when they built the EBR's.

Note: The readings I have listed as the what may now be the "Old" common lengths were engraved on headspace gages of various manufacturers for decades.

OK, let's add in the GI 7.62mm Headspace Gage Readings

.308 Win. GO 1.630"

7.62mm Go Gage........1.6335"

.308 Win. NO GO 1.634" - I guess we can now say this was the "Old Common Length."

.308 Win. NO GO 1.636" - I guess we can now say this is the "New" Common Length?

7.62mm NO Go Gage.....1.6375"

.308 Win. FIELD REJECT 1.638" - This has been THE standard or common length for decades.

.308 Win. FIELD REJECT 1.640" - New Length SOME manufacturers are using. This may have been the result of the GI Rock Island Arsenal going to that for the EBR? It ALSO is the SAAMI standard Field Reject limit for .308 Win.

7.62mm Rebuild Maximum Gage......1.6415"

7.62mm Field Reject Gage.............. 1.6455"

Now we have to remember that those in charge at some Headspace Gage companies are not the same people as in the 60's through 80's. I think Dave Manson used to work for Clymer? I think the gentleman running Clymer now is not the same person as in the 90's?

HOWEVER, what it looks like to me is the more current Headspace Readings on .308 for both the NO GO and Field Reject have gotten CLOSER to the standard GI 7.62mm Headspace lengths (though the 7.62mm Field Reject is still significantly longer than the SAAMI .308 Win Field Reject reading).

What I THINK the reason for this is because .308 Win. gages in the 50's through at least the 80's were used much more often in Bolt Action rifles and the newer readings reflect more use in Gas Operated Rifles where both .308 Win. and 7.62mm ammo are used, as long as it stays inside .308 Field Reject readings and does not go to the 7.62mm Field Reject reading.

OK, so in a few nutshells, what does this all mean for use in Commercial M14's and M1 Garands chambered for .308 Win./7.62mm?

The BARE MINIMUM GO Headspace for .308 Win. is 1.630", BUT in a Gas Operated Rifle where you are going to use either .308 Win. or 7.62mm you need minimum headspace of 1.631" for the rifle to operate properly as it gets dirty from firing. With NM or target grade barrels we use 1.631" to just over 1.632" as the minimum chamber and that is also "OK" for most GOOD 7.62mm ammo.

Now for NM or target grade barrels, most NM Armorers and Gunsmiths use a special "NM or Target" Reamer that is "tighter" or just at the minimum specs. That is fine for .308 Win. cases and 7.62mm cases that are QUALITY made and from the older U.S. ammo makers like Winchester or Federal. HOWEVER, SOME cheaper 7.62mm and some foreign surplus 7.62mm will not function quite right in a chamber reamed with such a tight chamber. SO, if you want to use more types of 7.62mm ammo, then the chamber should be reamed with a STANDARD reamer. MOST commercial M14 rifles have been reamed with a STANDARD reamer, BTW, UNLESS it has a NM or Target barrel. Again, if it has a NM or Target barrel, the cheaper 7.62mm and some foreign surplus 7.62mm may/will not function quite right in those barrels.

Now, for a rifle that you are going to use primarily cheap ammo or foreign surplus ammo, then a minimum chamber of 1.633" to 1.634" MAY be your best choice and in my opinion, it is. That little extra room in the chamber will be more forgiving of cartridge cases that are just not quite right in overall dimensions. When you are shooting this kind of ammo, you are not (or should not) be expecting the absolute best accuracy because the AMMO is not capable of it. Also, a chamber in this length is still going to give good accuracy with good ammo that MOST people will be able to get out of a rifle.

The SAAMI Spec for .308 Win. Field Reject or MAXIMUM LENGTH chamber is 1.640." If your bolt won't close on that length Gage, then you are good with both .308 Win. and most 7.62mm ammo.

However, if the bolt CLOSES on a 1.640" Gage with NO additional friction from the gage, then that chamber is too long by SAAMI safety standards. This doesn't mean the rifle will blow up, but the cartridge cases can over stretch and rupture/crack/or stick in the chamber. AS LONG AS the bolt will not close on a 1.6455" GI Field Reject Gage, then good 7.62mm ammo should be used in that rifle and SOME good U.S. Win .308 made with good cases will work. You just have to try the GOOD Win .308 ammo like Federal or Winchester to see if you will have these problems and if you don't have them, you can use them. (The reason for this is it seems Federal and Winchester make their .308 cartridge cases to the standards of 7.62mm cases.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #42 ·
...that was a fast reply...!!!...thanks for getting back to me...

...this is what caught me up on the first page...it seems in conflict to what you are saying here...

...I figgured I would have to forsake NATO spec loads and aquire a supply of the afore mentioned NM ammo...

...I had looked forward to aquiring some M118 ammo but remain unclear on whether or not it is suitable for my rifle...

...the load that showed flattened primers and slight cratering was PMC Bronze 147grn 308 that was also marked 7.62 NATO on the box...

...I have a small supply of Federal 308 WIN 165grn Bonded SP "Deer Thug" loads and some Hornaday 308 WIN 150 grn SST loads that looked like they would feed well through a gas gun and deliver superior termanal ballistics in various hunting situations...

...now I am having serious doubts aboot me ammo supply...live and learn, eh...???...
The problem comes in when we talk about the use of ALL 7.62mm ammo including poor quality and some surplus ammo VS GI or good U.S. made 7.62mm ammo. Folks on this forum use all types of 7.62mm ammo and that's where the problem comes in.

While I was a NM Armorer for the Marine Corps for some 23 years, three of the four types of ammo we used for NM M14's was 7.62mm. We chambered the rifles with tight NM chamber reamers and settled on minimum headspace of 1.631" to 1.632" as that ensured better function. Now, these were TARGET guns that were not used in combat conditions and especially not with POOR grade 7.62mm ammo.

The first two loads were Lake City NM in 7.62mm in both 168 gr. and 173 gr. loadings. (Now we have to remember that National Match Ammo is made with MUCH tighter specifications than even standard US GI 7.62mm ammo and WAY above poor grade or much surplus 7.62mm ammo.)

The third load was what we called "Mexican Match." I know we made this from 7.62mm ammo, but I confess I don't remember specifically which ammo. I know in some cases they just pulled the original bullets and replaced them with 168 gr. and 173 gr. NM bullets. In other cases, they pulled the original bullets and replaced them with 168 gr. and 173 gr. AND put a different powder charge in them. I also don't remember if they resized the cases before they put the new bullets in place, though I think they did in at least some cases and maybe most of the time. I do remember we resized the cases in the 70's at least. IF and when they resized the cases, they always resized with .308 Win. sizing dies, so that meant they would fit the tighter chambers. I was not an "Ammo Tech," so I did not get involved with loading/reloading ammo. However, this ammo was regarded more as "practice ammo" and generally was not as accurate as our other loadings.

The LAST load we used was a proprietary load that Federal loaded JUST for the Marine Corps for about two decades and it was labled .308 Win. We allowed the FBI and DEA to use it and I think we allowed one of the Canadian Special forces to use it, but no one else to my knowledge. Finally Federal asked us to allow marketing it on the open market and it is now known as Federal Gold Medal Match Ammo.

Now, we tested each NM M14 with ALL four loads to see which one or ones would shoot to our standard of a TEN shot 3 inch group (or less) fired from our super expensive test racks at 300 yards. NO rifle shot all four types of ammo to that spec and it was/is unreaiistic to think it even could happen. A TINY percentage of rifles shot the NM Lake City Loads and the Federal Load that well, but it was no more than one in maybe 30 or 40 rifles and was quite unusual. A good number shot of rifles shot one of the Lake City loads and the Federal Loads. A small number of rifles shot ONLY one type of ammo well and it was almost always the Federal .308 Win. load when that happened.

OK, now on to using the M118 ammo. Originally that was loaded TOO HOT for M14's and they down loaded it so it could be used in DMR's, EBR's and other "special use" M14's. The M118 ammo that is now generally available can be fired well in most GI and commercial M14's because that ammo is more tightly controlled than say Machine Gun ammo. If it functions in your rifle and shoots good, then use it. If it doesn't shoot well, then I suggest pulling it apart, resizing the case in a .308 Win. resizing die and reloading it.

OK, now as to PMC Bronze 147grn 308 that was also marked 7.62 NATO on the box. I really don't like PMC ammo as it can have a tendency not to function right or accurately in some rifles. Some folks have good success with it while others do not.

SO, what I recommend as "standard" plinking/practice ammo for commercial M14 rifles is either Federal American Eagle or Winchester White Box Q3130, 7.62 MM NATO, Full Metal Jacket, 147 GR ammo. BOTH of these loads work well and give good accuracy in virtually every commercial M14 I've ever seen or heard about.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #43 ·
...well, the deer thug loads seem to be a bit hot bit not so bad as the PMC broze load in this rifle...

...I am wondering if laping the bolt will make this rifle a long range walter mitty rifle w/o messing up the accuacy too much...

...I am thinking the quality of the bore and the bedding and other stuff might have more bearing than how tight the chamber.is...
How much you might gain from lapping the bolt depends a lot on how much bolt lug to receiver contact you have now. If you have at least some contact on the left bolt lug now, I would not expect much gain from lapping bolt lugs futher in most cases. It really is hard to try to do such a diagnosis when the rifle is not in hand.

· MGySgt USMC (ret)
7,047 Posts
Discussion Starter · #46 ·
Gus, I have a Fulton Armory M14 7.62. I don't have any headspace gauges to check it, but was wondering if it would be safe to shoot 308 ammo in it. Thanks for all the info you post. Very educational.

Well, I can never tell you with the absolute certainty of chiseling it in stone as there is a one in a million chances that something could go wrong because none of us are perfect when I can not check the headspace myself, BUT Fulton chambers their rifles so the minimum and maximum headspace is correct for .308 Win. So I would have no problem recommending you fire .308 Win. ammo in it that is designed for "Gas Guns." IOW, the Federal American Eagle ammo would be the brand to buy to make certain of it.
1 - 20 of 21 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.