Slamfire asks why are not off axis firing pin strikes detrimental to accuracy.
Because, if they're not too far off, they're very repeatable from shot to shot. The flame output and temperature paramaters are, for all practical purposes, the same from shot to shot.
If someone want's more details and the nitty-gritty technical reasoning, contact a primer making company then ask them. I've only offered my own experiences and reasoning.
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master