I don't think the firing pin helps, because the expanding case fills the chamber and presses back into the boltface. I guess an experiment could help.I think a 2 to 3 ounce firing pin pushed near 20 fps by a 25 pound spring force would very tightly press the case shoulder into the chamber shoulder. Sometimes setting the case shoulder back a couple thousandths.
Is the back of the chambered case body ever pushed against the chamber by the extractor claw?
Take 5 cases and set the shoulder back in .004" increments. Would they all fireform or would they separate?