Shoulder bump on a semi-auto

[fatelvis]

To all of you who reload for Precision shooting with an AR or M1A;
Do you use the brass after the first firing to determine the .003” shoulder bump, or do you have a different way of figuring your amount of bump? I use the stripped bolt method for my bolt rifles, but it doesn’t seem to work as well with the semi auto rifles. Thanks guys.

 

[mac86951]

I do use the fired brass; i picked up an RCBS precision mic; very useful tool. I doubt two copies are calibrated to read the same, but excellent for dialing in sizing dies.

Semi's can be known to "stretch" the brass during extraction; bit really only if your gas system is off timing, or shooting hot loads.

Don't forget the pause when the case is fully inserted into the sizing die.

-Mac

 

[edwardware]

In my M1 Garands. . . I once wondered me a wonder, and neck-sized and manually loaded for a few cycles to check.. My conclusion is that with mid or hot (for an M1) loads, the brass actually stretches (barely) beyond chamber dimension due to early unlocking.

I now FL size to fired -0.003, and they chamber without any "feel" in my M1s. The brass lasts 15+ cycles with neck annealing, and dies for loose pockets.

In my experience, ARs do not do this.

 

[Richard Coody]

On my precision ar's I pull off the upper and try chambering fired brass with just finger pressure on the bolt carrier. If its tight i minimally size and try again. this is the easiest way I have found to ensure I have fully fireformed brass before setting my shoulder bump. I use .003 for my shoulder bump. Now this is dedicated brass for this rifle only.

 

[Longtrain]

One of the benefits of annealing after each firing, you have much more control on bumping the shoulder accurately, especially for semi’s. The risk of a slam fire must be eliminated, for my M1, .003 is what I look for, and go as far as dropping the resized cases into the chamber to be sure they don’t bind anywhere. Sure, it takes a bit of time, but a CMP match is 30 rounds. No big deal, never had an issue taking the time to make sure everything is good to go.

 

[NorCalMikie]

Neck size for a semi auto ONLY if you know what you're doing and why.
I have done it with good results and knew why AND what to expect when I was done.
Pretty easy to just full length resize the brass and go shooting rather than go thru the bump process.

But you have to watch out for the "purists". You can't do anything! It'll never work!
You'll shoot your eye out!
Ran into a situation with my M1A years back. Pretty new to the semi auto.
Was running a bunch of rounds thru it D) and had a couple of extras laying on the bench. Pulled the empty mag, stuffed one round in the chamber and tripped the bolt release. BANG!! Floating firing pin did it's deed. The guy on the next bench said "I bet you won't be doing that again". He knew exactly what happened. Lucky the rifle was still pointed down range.
I happened to mention what happened on the M1A forum and right off the bat, I got the usual, "You'll shoot your eye out". "You better send it out and get it checked by a Gunsmith before you even think about ever shooting it again". "It'll blow up if you shoot it and you'll kill yourself and anybody else around you".
That was about 1000 rounds ago and so far, NO PROBLEMS.
And one of the purists said "YOU CAN'T SHOULDER BUMP SEMI AUTO BRASS". Says who?? That's where you start playing IF you know what you're doing.

 

[Richard Coody]

Neck size for a semi auto ONLY if you know what you're doing and why.
I have done it with good results and knew why AND what to expect when I was done.
Pretty easy to just full length resize the brass and go shooting rather than go thru the bump process.

For just shooting ar rounds i don't even bother to reload.any more. You can buy 1000 rounds of lake city bulk ammo for less than $400.