M1A Headspace Issue / Question


Welding Rod
February 13, 2011, 10:53 PM
I have a SA Standard M1A. It was built in 06 or 07. I bought it new. It has approximately 2-3 hundred rounds through it. It has only been shot with factory ammo. The gun is completely stock.

There was a SA slip that came with it that noted the headspace was set at 1.631", that is one thousandth above minimum.

Today I was gauging some brass for headspace with my RCBS Precision Mic. I believe my the gauge to be accurate as most unfired factory ammo is gauging from the .308 maximum allowed dimension at - .002" to .000".

Brass fired through my Ruger Hawkeye .308 is gauging + .001" to +.002" after firing.

Brass fired through my SA SOCOM is gauging +.005" to +.007" after firing.

But brass fired out of my SA Standard is gauging +.013 to +.015" after firing.

This is with a couple of different commercial US .308 loads and South African FMJ.

I have checked to make sure there is no primer protusion or burrs on the case head before measuring.

Now an odd thing is this stretched brass that comes out of the Standard will not rechamber back in the same gun. The bolt will not go forward enough to allow it to rotate into battery. I guess that is the good news. Maybe the headspace is OK afterall (?).

So any idea what is going on here? Why is the brass being stretched so much? Can I prevent this? I would like to be able to reload the brass, but that level of stretching seems really excessive.

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February 13, 2011, 11:14 PM
What you are seeing is not abnormal. In fact why the SOCOm is not stretching the brass/shoulder forward a much as the Standard is more the mystery (to me) than the numbers you are seeing from the Standard.

Please know that that M14 design is very rough on brass as it extracts under fairly high residual pressure. That pressure has still got the capability to blow the shoulder forward more than you might think during unlocking -- hence the numbers you're seeing.

See: http://www.m14.ca/reloading/14_loading.pdf

"For one, the M14 has about the quickest operation
of any auto-loading rifle — so much so that
the bolt can unlock before the case has fully
retracted from its expanded dimensions. We’re
talking about milliseconds, but that’s all we have
devoted to the entire process. The net result is that
some amount of additional expansion frequently
occurs in a case fired through an M14." .....

One thing an M14 owner absolutely, unequivocally
must accept is short case life. A few things
may shorten it even further, but nothing extends it.
Never allow more than four firings on a case."

Welcome to my world.... :)

Welding Rod
February 13, 2011, 11:59 PM
MEHavey - Wow that is exactly the info I needed.

Thank you very much!

February 14, 2011, 01:21 AM
I reload 7.62 fired in a real M14 and I scrap it after 5 reloads, without fail. I sorta envy the guys who claim they can get 15 reloads or more out of a piece of brass.

February 14, 2011, 01:23 AM
While some M1A's do stretch brass, some don't. I have both a Standard and a National Match, and neither of them stretches brass. I even closed the gas port valve on the Standard to see if there was any difference between firing it as a bolt action, and firing in the semi-auto mode, and there wasn't.

If you want to test the headspace of your rifle, just close the gas port valve and fire five rounds manually. You'll have to work the op-rod handle the same as a bolt action rifle to extract the brass, but it will tell you what your rifle's actual headspace is.

It could be too that the powder used in your factory ammunition isn't optimal for the M1A's gas system. I load mine only with IMR 4895 powder.

Hope this helps.


February 14, 2011, 07:39 AM
I would strongly recommend using a small base die and limit reloads 3-4 times. Use LC brass if you can find them. Keep cases trimmed to 2.00 to 2.05. Never use Fed primers. CCI #34 would be best. IMR 4895 is the correct burn rate for operating system. I learned this the hard way. After having a couple of slamfires with my Socom 16. I sent it back to Springfield to replace the bolt and rework the chamber. I believe my slamfires was due to excessive reloading and using a FL die...

February 14, 2011, 08:16 AM
As mentioned earlier, your rifle is stretching the brass because the action was designed to open before barrel pressure is zero. The term, in Col Chin’s book, is “residue blow back effect”. This is done to provide a little bit more time when there is useful pressure to activate the mechanism. The pressure cannot be so high that the case is burst.

This is another reason why gas guns are limited in the powders, pressures, bullets they can use.

The inset of this pressure curve diagram, which is from AMCP 706-260, shows the portion of the curve which unlock occurs in the 308. Chin states that unlock occurs under 650 psia for 20mm cases, so I guess unlock is sometime after 3.3milliseconds.

Some rifles, just due to variance in manufacture, will open up sooner in the pressure curve. Case life will just be awful for those rifles. I have one, had to go to a vented gas cylinder lock as it was stretching cases too much.

But either way, early in the curve or late in the curve, gas guns stretch brass.


Also as you noticed, these gas guns don’t have massive closing cams like bolt rifles. You read all the time advice from bolt gunners about partial neck sizing or just neck sizing which they claim will extend case life, improve accuracy, etc. These sloppy reloading techniques will work in a bolt rifle as those mechanisms have massive cams and leverage. Bolt gunners who follow these practices are just crunching cases to fit in their chambers as they beat down the bolt handle. They do not notice any resistance above that of mainspring compression until finally, the case work hardens so much that they have to full length resize to get the bolt to close. Attempting to copy their practices reloading for a gas gun will result in jams, and worse, in a M1/M1a, out of battery slamfires.

For gas guns I recommend small base dies and setting up the dies with cartridge case head space gages, such as you are doing.

I set my case shoulders back about .003", or I simply size to gage minimum.

Welding Rod
February 14, 2011, 09:05 PM
Thanks for the input. I will try shooting the gun with the gas valve closed and see what happens.

February 14, 2011, 11:09 PM
I carried the M14 while in The Marine Corps. The gas valve is for shooting rifle grenades with it closed. This was so all the pressure could be used to propel the grenade and the back pressure wouldn't drive the bolt back too fast.

The only thing that will happen is the rifle will act like a bolt action rifle, and the cases will only stretch as much as the chamber will allow.

Hope this helps.


March 12, 2011, 06:59 PM
question?if you closed the valve down a bit,would you be able to slow down the action/pressure to a point it would not ruin the brass?

March 12, 2011, 08:13 PM
The gas valve is either open or closed. It's not adjustable.

Neither of my M1A's ruin any brass.

Hope this helps.


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