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M1A and Odd Headspace Question

5K views 34 replies 15 participants last post by  canmancan 
#1 ·
This is a follow up to an earlier post. My M1A is stretching brass like crazy and I wanted to know why. Now I am really stumped:

My M1A's bolt won't quite close on a 1.638" headspace gauge (Forster .308 Win field gauge). The gauge goes easily into the chamber.

I used my RCBS Precision Mic (.308 Win) to measure both the headspace gauge and the brass that I fired in the rifle, with the gas valve shut.

The fired brass measures .006" longer than the headspace gauge that the bolt won't quite close on.

Couple of details: Like new rifle probably has less than 500 rounds through it. New Black Hills Match ammo was used when shooting with the gas valve shut. The bolt was stripped, and all parts, chamber, and Precision Mic thoroughly cleaned before measuring. Headspace gauge is brand new and there are no burrs on the brass.

With the gas valve open the brass stretches an addional .003". Various loads stretch about .013" to .018" total on one firing depending on if the gas valve is open or closed.

Any ideas how my fired brass is coming out substantially longer than the chamber when I am shooting with the gas valve shut and hand cycling the action?
 
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Discussion starter · #18 ·
jmoore - Thanks for tip. The stretch I am measuring and have indicated (.013" to .018") is in the head space dimension of the fired brass, not the overall case length. I am measuring the headspace dimensions of the cases before resizing.

Jbrooks - I will try that. Flex sounds logical.

Also sounds logical the shoulder area of my chamber may not be flat or true to the bolt face.

In either or both cases, bummer as it looks like there will be no way to get any life out of the brass. If it was a headspace issue at least I could have gotten it fixed by SAI.

But, this rifle does seem quite accurate, shooting 1 1/2" 5 shot groups with match ammo so on the other hand I really didn't want to replace the barrel.
 
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