It's hard to answer your question because of the variations that are found with the tools, the chambers, and the cases. is a strong possibility and that basically you are looking at a shot out barrel...
I made my own HS guage using a true 0.400" ring guage and have the same results with My M14 clones. The fired cases, with port closed, measure longer than I would suspect. But tolerances in chamber angles and a little elastic "give" to the bolt under 50KPSI pressure will result in cases that "appear" stretched.
Try this: After firing, with port closed, pull the bolt back about an inch and let it go forward slowly to rechamber the fired case. You will note that the case does NOT slide easily back into the chamber. It has stretched and as also stretched the chamber and bolt lockup which will rebound and trap the case.
So, Every case will APPEAR to be stretched, leading you to think that you HS is off. But it's nothing to worry about. Measure your HS with a real HS guage and let it go. A couple thou are of absolutely NO concern.
If you unscrew the sizing die a tad (0.01"...) you will end up with a case that is Mostly resized and will lengthen your case life.
The DOWNSIDE of this is that the case will probably NOT just simply drop into the chamber friction-free.
But, for non-combat/life-threatening situation (i.e., paper punching) this should work. The force of the bolt slamming forward will ensure proper chambering.
I have actually found there to be NO measureable difference in group size using this method, but case life will improve slightly.
Bear in mind that most ammo (even milsurp) is right around 1.630" HS to begin with and a NATO chamber minimum is specified to be 1.6355". So a round fired in even a minimum chamber will expand at least 0.005" plus probably 0.003" to make up for the flex bwtween the bolt and the chamber. Since your chamber HS is 1.638" +/-, you are loking at a minimum of about 0.01" stretch in any case.
We all have to admit that the design of the rifle is as a Combat weapon, not a match rifle. So we have to put up with some of the side effects like shorter case life and possible feeding issues. Otherwise use factor ammo ro resize to factory specs. (BTW, I have found many dies will actually OVER-resize, that is, they resize to Less than factor specs. so you have a lot of wiggle-room.)
JWB
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