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A little bit of everything to do with headspace.

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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.
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MGySgt USMC (ret)
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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.
 

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MGySgt USMC (ret)
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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.
 

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MGySgt USMC (ret)
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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.
 

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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.
Thanks Gus, You answered so many questions for me before I even knew what to ask.
 

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MGySgt USMC (ret)
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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.
 

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WIN Nato Whitebox

Just last week I shot this ammo thru my Gus rebolted M1A and it worked great. Better yet, it is available at Midway as we speak:

http://www.midwayusa.com/product/72...cket-box-of-20/?cm_cat=PAN&cm_pla=ProductDesc

It is pricey but available and it shoots as good as the surplus Portuguese NATO I have been using. Move quickly if you need some.

Another option is the Privi .308 that Midway often has. It is a bit cheaper than the Win NATO but is comparable in accuracy. A few weks ago I ordered and received a 200 round battle pack for somewhere around $170. The primers are sealed.

I have shot the Privi as well as NATO in my M1 Navy Mod 1 and both work well.

Brownells has the Clymer .308 field max gauge (1.640") and that is what I use to check the headspace with. Also have the Clymer 'Go' gauge and use that as well.

Hopefully the CMP will be sending me another M1 Navy BR soon so I can put the gauges to work again.

Rich
 

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MGySgt USMC (ret)
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Discussion Starter · #49 ·
Folks, I don't know WHY I keep messing up when I type the reading for the 7.62mm Go Gage INCORRECTLY as 1.6355", but that is wrong.


The CORRECT READING FOR 7.62mm Go Gage is........1.6335"

I just went back and corrected it in my earlier and more recent "all encompassing" post on Headspace for both Win. .308 and 7.62mm headspace. My apology for confusion this may have caused.
 

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This continues to be an informative and timely topic.

On 5 July I received another Navy 7.62 Mod 1 barreled receiver from the CMP. My first one last winter was chambered for 7.62 and the bolt I had would not close on a 1.638 field gauge so no problem with using .308 ammo or 7.62.

The new BR however is chambered really long. Fortunately I acquired a 65 series bolt (late post war I believe) from Gus last winter. This bolt will close on the 1.638 field but not on the Clymer 1.640 SAAMI max gauge... whew! Based on the SAAMI gauge this rifle will be safe to use with .308.

As word of caution, if you are planing to assemble a Navy 7.62 M1 BR be aware of the long chambers. If you are planning to aquire only one headspace gauge the Clymer SAAMI .308 max is probably the one to get as it will give you assurance that 7.62 NATO and .308 are both safe to use.

Rich
 
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