Overcrimped/Slamfire on M1A

[Unclenick]

Slamfire,

Your posts in this thread are great, and I hated having to edit the last one, but it appears to run afoul of the board policy on posting copyrighted materials. You need to get explicit permission from the board where it was posted (they have the implicit copyright) and then put into the post that you have that permission along with the material. If you get that it can all be put back in (though following the link will still let a reader see it all now). I hate having to enforce the policy on an informative post, but in a world of lawyers run amok, ignoring it is not an option moderators are allowed.


NinjasHateCommies,

I think Bart is right that your "slamfires" are not really slamfires, but are most likely to be a rebound doubling from how you hold the rifle. To understand how this happens, with your gun empty, hold it sideways and slowly press the trigger until the hammer falls. Then slowly release the trigger until you feel (and usually hear) a click where the rear hammer hooks let go and and the front hammer hooks jump up to meet the trigger. This is the amount of movement needed to reset the trigger to fire another shot. If you hold the gun loosely enough, recoil will back the gun up that much relative to your trigger finger position and before your trigger finger pulls back far enough to prevent it. That resets the trigger. During recoil, the butt of the gun compresses your shoulder tissues, and afterward the elasticity of those tissues pressing against the butt bounces the gun back forward. This forward bounce pushes the now-reset trigger forward against your trigger finger, firing the gun again. This all happens faster than the 0.3-0.6 seconds it takes a human being to react to something, so it's not under conscious control. Occasionally, even a triple can happen this way, but it usually does not because the muscle tension in your trigger finger continues to act as a pulling spring, causing the finger to progress rearward after the initial trigger break, so that it gets far enough to the rear to prevent a second occurrence in a row. The first two shots take long enough to occur to allow that to happen.

The cure is to prevent the initial compression and bounce from resetting the trigger. The easiest way to do that is to pull the gun firmly into solid contact with your shoulder. That is what a tight sling does. It pre-compresses the shoulder tissue so there isn't much rebound bounce. In prone and sitting you can also use the pistol grip to help the sling pull the gun into your shoulder if you can do that without introducing tremors to your sight picture, but the tight sling is more reliable as it doesn't fatigue. I know of no standing offhand position I like that makes that pull into the shoulder practical to do without introducing tremors into the sight alignment, so I simply always fire offhand sighters single-loaded. I strongly recommend you do the same.


Regarding your case separations, you seem to have a handle on that. Way too much shoulder setback during resizing. Where they are separating is a little further forward than we usually see. This seems likely to be due to the brass being made thicker than usual in the walls just forward of the head. I've seen some sectioned foreign brass made that way, and I've heard (not verified personally) that Remington gets some brass made on contract overseas. We'd have to section a case to find out.

I am still struggling with how you were able to to size cases -0.012" to -0.015" below chamber minimum. A correctly made standard sizing die with a correctly made standard shell holder simply should not be able to size brass that small. It sounds like either the sizing die is too short or the shell holder deck is not the normal 0.125" thick above the part of the slot the case head rests on. The shell holder can be measured with a caliper. Simply measure the overall height of the shell holder. Zero the caliper, and then measure from the bottom of the holder to the surface the case rim rests on during sizing. The difference should be -0.125" nominally, though a couple thousandths off is not unheard of among some makes.

If your die is short, you can buy a Redding competition shell holder that is +0.010" extra high above the case supporting surface to compensate for that.

Your cases coming out of the chamber +0.005" is pretty normal for a NATO spec chamber. A SAAMI GO gauge is 1.630", while NATO is 1.635". Allowing for springback, your chamber is probably actually about 1.636"-1.637", and you want your cases about 0.004" smaller for trouble-free chambering, so about 1.632"-1.633" after resizing.