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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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Eye Master
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Gus,

Your thread is timely. I just tried making some snap caps for a 30-06 on a CNC lathe. I programmed them to SAAMI specs, and damn if the bolt would not close.

I then looked at the SAAMI chamber specs, and was amazed to see that there was interference between the SAAMI case headspace dimension, and the SAAMI chamber dimension. According to the published drawings on the SAAMI web site, the case chamber dimension is 2.0526" +0/-0.0070", while the headspace dimension for the chamber is 2.0487" to 2.0587".

In this case, a large case at 2.0526" will have around 0.004" of interference in a low end chamber of 2.0487"

What gives?

I could not figure it out, so I looked up SAAMI drawings for .308 and .223, and they have a similar overlap:
.308 has a case of 1.634 -0.007 and a chamber of 1.630 to 1.640.
.224 has a case of 1.4666 -0.007 and a chamber of 1.4636 to 1.4736.

Double 'what gives'!

Thanks for any thoughts on it.

Art
 

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Eye Master
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4,272 Posts
Thanks for the detail. No doubt this difference between military and SAAMI is adding to the confusion, but I don't know if that is the problem. FOr one, your link suggested military chambers might be longer than SAAMI, so if I make a SAAMI snap cap, it ought to fit easily in a Garand chamber - but it does not.

I will go back and verify my shoulder angles, but I think they were OK. I could see that it was the edge where the case body joined the shoulder that was interfering, so if I keep the same neck, and just incline the shoulder back slightly, it might help, though again, this does not explain why the SAAMI chamber and the SAAMI case have an interference, just because when you look at the headspace numbers, the case is bigger than the chamber. Too strange!

Art
 

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Eye Master
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4,272 Posts
Jay,

1. Thanks you for that link. It is an excellent find, and confirms what I had seen.
2. WTF? THey come right out and say there is an interference inthe stack-up of a long case and a short chamber, and come right out and say that this can cause the bolt to not close ..... but then continue on, as if this is no big deal?

The only thing I can think is that SAAMI is taking the safety perspective, and the size range they are giving will produce a safet cartridge. They are not looking at good function - that is up to the manufacturer. If you look at the SAAMI specs, it suggests there is about a 0.010" range of lengths that will work. THe Armalite article suggested that an acceptable range for function was closer to 0.004" for a rifle, 0.003" for a machine gun, and 0.002" for a NM rifle.

My solution was to shave about 0.006" off the snap cap body length. I'm awaiting samples, but hope this resolves the issue.

In summary, it seems to me that a lot of this confusion on headspace is exactly what is going on here - different people are trying to do different things with these numbers:

SAAMI is looking at safety, and trying to account for wear.
Mil-Spec is looking primarily at new gun production, and possibly accounting for less frequent cleaning.
Manufacturers are looking at targets and tolerances that will work.

Add to this that we are discussing 60 year old rifles, gages and jigs that were built before ISO 9000 and calibrated standards, and that people misunderstand the intentions of numbers, and mis-apply them, it makes for a good confusion soup.

One would think that with today's CNC equipment, and calibrated standards, SAAMI could come out with modern target dimensions that make a little more sense. After all, these days a +/- 0.005" tolerance on a machined part is very, very, sloppy.

Thanks again for your help.

Art
 

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Eye Master
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4,272 Posts
Interesting, I got automatically censored. When I typed a W, followed by a T, followed by an F, and posted it, the system automatically substituted "what the hey".

I guess the initials for a bad word are now considered a bad word... hey, that's a BAD LETTER. as in "I'm sorry Mrs. Johnson, we had to send little Johnny home from school today, because he used a bad letter".
 

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Eye Master
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4,272 Posts
Gus,

I made them to the max SAAMI dimension (ie the dimension on the drawing, which was +0/-0.010), and they did not fit in my Garand.

I have now shaved 0.005 off the shoulder, and they fit.

My shoulder angles were 17.5 degrees, which is the SAAMI spec.

As I mentioned in my previous post, I suspect SAAMI drawings are max safe dimensions, which does not necessarily mean these dimensions are optimal.

Art
 
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