"And shooting a factory round in a AI 30/30 chamber with no locking lugs, shows very little breech pressure occurred. It should have been greater (with an unformed case), due to the case having to expand into the chamber before moving to the rear."
This sounds like you're saying the pressure from a factory .30-30 round in the AI chamber should be greater than when the ssame ammos' fired in a standard chamber. Is that actually what you're claiming?
I can see primers backing out of a .50 BMG case because the brass is so thick it resists stretching. Have seen the same thing with other cartridges using very thick brass, including the WSSM's and .338 Lapua.
But in standard commercial cartridges like the .270 I have yet to see it--with one exception. I'm often sent rifles ammo for test reports in various magazines. Consequently I have a bunch of .270 factory ammo on hand, the brands including not just the usual Federal, Remington and Winchester, but Hornady, Norma, RWS and a couple of small makers. In most of those brands there's more than one load. Since January of 2014 I've fired the ammo in four of my own .270's and five others, made both in America and Europe, so they could be assumed to have slightly varying headspace.
I just checked the fired cases remaining in the boxes and none have primers backed out--EXCEPT some Hornady Custom Lite ammo loaded with 120-grain SST's at a listed 2675 fps, obviously a reduced load. The primers in these were backed out anywhere from .003" to .007". The lack of backed-out primers in the full-power ammo, fired in various rifles, does seem to be contrary to your experience.
