On the JP there is not upper and lower step..I called them and according to the guy that answered the phone the guage does not check headspace.
I wonder if the representative meant check
chamber headspace.
For bolt guns, a fired case blows itself to the chamber and stays chamber dimensions after extraction. Whatever shrinkage actually occurs is below anything I can measure. If you have not noticed, by the time your hand reaches the bolt on a bolt gun, barrel pressure is zero. Barrel pressure drops to zero in milliseconds.
However, gas guns unlock when there still is residual pressure in the barrel, and this is deliberate. Designers want enough residual pressure to help “pop” the case out of the chamber. Obviously they don’t want so much residual pressure that the case separates or ruptures, so timing is very important and studied.
These are from Chin:
So, because your AR15 actually extracts while there is pressure in the barrel, any case that comes out of that barrel will be stretched, and it won’t be the exact size of the chamber.
These are 308 Win cases I fired 23 times in my match M1a. I lubricated these cases and never experienced a case head separation. Many shooters experience case head separations in under five reloads in these mechanisms with dry cases and dry chambers. What happens with dry cases and dry chambers is that after the primer is struck, the case neck and shoulder fold out and adhere to the chamber walls, than as pressure builds, the midsection of the case is stretched to the bolt face. With lubricated cases, after the primer is struck, the case slides to the bolt face because the friction between chamber and case is so low that the case neck and shoulder cannot adhere to the chamber walls. This eliminates case sidewall stretch.
But, the shoulders are affected during extraction. Here I measured the shoulder to base distance on these lubricated cartridges and found just how much that shoulder distance had moved during extraction.
My lubricated cases were sized to 1.630”, which is the minimum dimension on a Wilson gage, but on extraction, they were longer. What I believe is going on is during extraction, during that low pressure residual extraction period, the shoulders are still under pressure to fit the shape of the chamber, but the chamber is growing longer! So, attempting to measure chamber headspace by measuring the base to shoulder measurement of a fired cartridge case, is
doomed to failure in any gas gun. It might work in a long recoil gun, it would have to be a mechanism where unlock and extraction occur when barrel pressures are zero.
This advanced primer blowback mechanism was used in the Polsten cannon, among others. Since this is a blowback, there is no locking mechanism (!) and there is a lot of travel in that chamber, I really doubt there would have been any case shoulder left on extraction. For this mechanism to work, in fact for all of these that were manufactured, the whole cartridge had to be greased!