You could be pushing the shoulders back too much during the resizing step. Reloader induced excessive headspace will get you flattened primers every time.
Upon firing, the firing pin will push the cartridge forward in the chamber. The primer ignites and starts to extrude. The building pressure will cause the case to expand in all directions with the thinner parts of the case expanding earlier in the cycle and holding the case forward in the chamber. The rest of the case expands later in the cycle because it is thicker and the solid head slams back against the bolt face and hammers the primer back in the pocket and flattens it. This will happen regardless of the pressure unless the cartridge is loaded to pressure levels low enough that the case will not expand. .
FWIW, this is the reason that you can't read pressure by looking at the primers.
Last edited by MichiganScott; 09/03/17.
