M1 Garand shoulder setback, 0.010" too much for a one time firing?

[Laphroaig]

My GCA (Garand collectors) Journal arrived in the mail yesterday, and the first article was entitled "Out of Battery Discharges". It was a forensic study of 2 recent incidents where OOB discharges cracked the receiver of the rifle, and in one case sent a small piece of shrapnel back into the forehead of the shooter.

I resurrected this thread because I feel some bad advice was given to the OP and maybe others that could damage their irreplaceable rifles and hurt them as well. That advice was to size casings to 0.003" smaller than the chamber, rather than to produce SAAMI spec ammo, for use in M1 Garand rifles.

The thread did morph into discussion of shoulder setback for bolt guns as well, which I have no problem with. If you can't close the bolt the rifle won't fire and you know your round is out of spec w/r to the chamber headspace. That's a big difference.

In both incidents cited in the article, handloads where the shoulder was not set back far enough to allow the bolt to fully close was determined to be the cause. (In both incidents the rifles and ammo were inspected by CMP armorers and in one case the gunsmith who built the rifle, so the findings are not purely speculation) The free floating firing pin apparently struck the primer with enough force to set it off, and the unlocked bolt slammed back to the rear of the receiver with enough force to crack it.

Trying for the minimal headspace in an M1 rifle is playing with fire. Should a piece of brass spring back a little more than expected after sizing, or a speck of dirt get in the chamber, the result could be an OOB. So I reiterate the responses given in Posts # 3 and 14, load SAMMI spec ammo that the rifle was designed for. Also be certain the primers are seated slightly below flush.

 

[Bart B.]

Trying for the minimal headspace in an M1 rifle is playing with fire.

So is basic reloading of any cartridge if you don't understand the mechanics of what happens, don't make correct and proper measurements and lack the basic machine shop skills and tools to do all the right stuff.

Minimal headspace has always produced best accuracy with Garands. Not everyone is qualified nor equipped with all the right stuff to do it safely.

All the two dozen plus slam fired cases I've seen had full, normal primer dents in primers. They were fired with the bolt fully closed. It takes about .020' deep dents to fire primers. Close inspection of all Garands involved showed too little sear-hammer engagement when cocked. The shock of the bolt closing disengaged the hammer and it fell normally on a fully closed bolt firing the round.

That said, a few are a fault of the mechanics of primer cup and firing pin plus safety bridge in the receiver. These instances do happen.

 

[Laphroaig]

There is a difference between a slam fire and an out of battery discharge. One occurs with a closed bolt, and is surprising but not damaging, the other occurs with an open bolt and can damage the rifle and perhaps the shooter as well.

Not everyone is qualified nor equipped with all the right stuff to do it safely.

My point exactly. Why press your luck?

 

[High Plains Drifter]

I agree that in a perfect world, ie. the world of the bench rest shooter, that a case should be sized for the chamber for which it will be fired from. The reason that SAAMI specs exist is for the average chamber since in combat you take the ammo you are given. You don't have the luxury of spending hours in your reloading room meticulously dialing in sizing dies to perfect dimensions. I use Wilson's case gauge, which helped me get my brass right to chamber in my M1A. This gauge is stepped and all you have to do is get the case sized between the 2 steps.

For a semi auto rifle you will have some variance from loading to loading in brass, I think from the conditions under which you fired a particular batch/load & brass manufacture, but as long as you always bring it back to SAAMI specs you should be fine. I would not push the shoulder back too far and keep it to no more than .005, but again as long as the brass fits in the case gauge between the steps you should be good.

BTW, I saw a post about 10 firings on cases? I would not go more than 5 for a semi auto, and preferably 3-4. I use Lake City or IMI cases only so that I keep variable in brass to a minimum, which again should be good for the Wilson case gauge to give you good and safe results.