Ok Bart, I'm gonna test this theory. BTW, I'm saying that shoulders don't set back from the firing pin strike. I'm just curious, I've never confirmed nor tested for such. I'm gonna push a fired primer back out to normal pre fired condition, fill the cup with some JB Weld, then measure a piece of brass head to datum line shoulder before, and after firing pin strike.
My primary reason for jamming, is simply based on my experience with shoulder setback, in that it allows the shoulders to reach my target forming dimensions, that being within .0015" - .000" of zero fit tolerance to the chamber. If it works because I'm defeating the effect of the primer impact, or because it defeats extractor button pressure, or defeats some other element of physics involved, I don't know, it just works.
But now you've got the science part of me curious, so I'm gonna remove the extractor button to eliminate that variable, insert a piece of measure brass that fits at zero, and see how much set back the primer actually creates, if any.
GS