You're correct that the case is driven forward by the fp, this is in all rifles, not just military ones...
Also correct in that primers become partially unseated at detonation. However, your primers should reseat after detonation and case head stretching.
The fact that your primers are not
reseating is indicative of excessive headspace in my opinion. Too much air between the end of the case and the boltface.
Your fired case head dimension is almost .01 larger than unfired brass- too excessive.
The fact that the brass didn't blow up doesn't mean excessive headspace doesn't exist- get a no go gauge. Brass is very forgiving- it can stretch a lot, and return to dimensions that can mask a problem- for a bit, anyway...
Good article on internal ballistics at Hornady: