I have seen the "mushroom" look of primers before, with very light loads. It is a head space issue, not a "hot" load issue in most cases.
What is "generally" happening is; The firing pin hits the primer and shoves the case into the chamber to the point it contacts the shoulders and forward movement of the case is stopped. The primer is ignited and is pushed back out of the the primer pocket when ignited and stops against the bolt face. At this point, the primer is sticking out of the case a few thousandths of an inch or whatever the head space slop is in the chamber.
Now the powder is ignited and the case is shoved back against the bolt face. Remember, the primer is sticking out of the case a few thousandths. The part of the primer cup that is sticking out of the primer pocket is hot and has expanded a couple of thousandths of an inch. It is now too large to seat back into the case, so it creates a little mushroom on the back of the primer, which flattens against the bolt face. The firing pin is fully forward at this point as well and often pierces the primer, so it looks like an over pressure load, when in reality it isn't. I hope all that made sense
Flinch
