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Old May 17, 2002, 08:17 PM   #1
Fatelvis
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Proper sizing for headspace

I just purchased the RCBS Precision Mic, and the directions seem to be aimed towards bolt actions. The instructions they give for proper sizing, without bumping the shoulder back too far, says to find your chamber dimentions, and bump the shoulder back .001-.002". I`m trying to reduce case stretch, due to a somewhat long chamber. Is that enough for a semi-auto, (along with full length sizing)? Has anyone used this for a AR or M1A? Thanx for any imput guys-
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Old May 17, 2002, 09:06 PM   #2
Art Eatman
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An important thing for semi-autos is the diameter of the case head--which is why the "small base" dies for the .308, and full-length resizing. You don't have the force in a semi-auto as compared to a bolt action to open the bolt after firing.

I sorta doubt that 0.001 or 0.002 would be too much stretch...

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Old May 17, 2002, 09:16 PM   #3
Fatelvis
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Art, I would LOVE to only get .001-.002" stretch, each firing. I had case head seps, because they were streching .012"! (That was before the Precision Mic, using only a Wilson guage, and not knowing my chamber specs) To clarify my question: Is .002" enough shoulder setback for my M1A, I realize it doesnt have the "camming" ability of a bolt action, to force the case into the chamber. Wouldnt a little Carbon, fouling, unburned powder, etc., getting in front of the shoulder, prevent the bolt from closing fully, with such a small margin of "slop"? Should I figure a little more space, to accomidate a buildup of "crap" in the chamber?
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Old May 18, 2002, 12:40 AM   #4
Archie
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Elvis, you're going to have to "fit" them.

Adjust your dies so a couple casing won't chamber. Then bump the shoulder back until they do fit. Just fit. You're right about the "camming action" problem, so you'll have to compromise some.

Either that, or don't use cases more than twice. But I'm way too cheap for that.

Try this: Load a couple rounds at starting levels, and seat the bullets so they engage the rifling. This way, you know the base of the case is bottomed out on the bolt face. Shoot those; you'll get a case that exactly fits your chamber. Now measure those couple cases and see how they have to be. Adjust your sizing die appropriately.

Or have the thing rechambered. Ouch.
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Old May 18, 2002, 01:41 AM   #5
uglymofo
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The RCBS Precision Micro die works fine if you have fire-formed cases to measure. Disable the gas cylinder on that M1A so the action won't cycle and fire off three rounds by hand-cycling the action. Save those cases aside and then measure them when you get home.

Another trick is (to pour mo' money at the problem and) buy the RCBS X-die. That'll reduce case stretching significantly- supposedly you can get 10-15 reloads just like a case used in bolt guns. I use a Wilson case gauge to check my fired M1A cases; if the case doesn't fit there after firing, it's stretched too far for comfort to resize; my logic being, before firing they dropped in with some slop, and now the stretch is so great they can't.

Small base dies are a necessity if your chamber's tight, but I don't think it's an automatic requirement for resizing for the M1A. My brass used to resize fine with the 'normal' RCBS FL die before I went to the X-die, and I was resizing for an SA Supermatch. Theoretically, the tolerances are better (read tighter) in that gun.

I've not been able to seat to the lands. My magazine is too short. I'd also be careful about allowing for too much 'camming' space. One has to be wary of slamfires. Check out Zediker's website for an article on "Reloading for the Match M1A" at

http://www.zediker.com/downloads/14_loading.pdf
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Old May 18, 2002, 09:32 AM   #6
Fatelvis
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Thanx Arch and Ugly. Ugly, that article you posted is THE BEST Ive read yet! It answers all my questions about bumping the shoulder for headspacing in a M1A, and alot more! I suggest that ALL reloaders/shooters of the M1A , read it very carefully. Great info!

Last edited by Fatelvis; May 18, 2002 at 01:00 PM.
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