For those of us that aren't professional primer seaters, What are the indications of 2-4 thou crush? Thanks
The indication is as measured.
The indicated K&M operation begins with
simultaneous zeroing of a primer about to be seated AND it's intended primer pocket. This, adjusting for variances in both primer heights and pocket depths. It's tarring the scale. Then when seating the primer, it's indicator displaying zero means the primer is at touching pocket bottom. This, regardless of force amount. Go further 2thou crush for Feds, 4thou for CCIs (my recommendation).
With this, each primer is preloaded/sensitized to a known good
standard.
If you desire those primers to always be below the casehead (they should be), uniform the pockets beforehand, with a preset Sinclair uniforming reamer.
At these standards you can then test & adjust your firing pin striking (with given trigger sear position) for tightest grouping.
Simply setting primers below casehead means nothing about their relationship to pockets. And neither does smashing them with great excess force or travel. As far as these seaming good enough, I've tested and found that primer striking can be screwed up to the point of doubling group size, and every single one still fired. This was with a 223, and was of course not the achievement to settle on..
That many 6PPC shooters are so sure they can tell anything here without measure, (by feel) isn't based on their primer measure (because most don't do this), it's their group measure giving them confidence. Then again, they're running underbores at competitive pressures way beyond viable for hunting cartridges. They're so deep in diminished returns from powder load, that
any firing of that powder is good enough to reach result. Primers will still fire, and pressure peak will still be way up at 70-85Kpsi for them. And a normal variance in that, their variance, is unlikely to show on paper -up close.