I think two things are happening.
I think his loads are too hot #1.
And he says he cleaned the action and found a piece of metal in the firing pin hole. And it's firing factory loads correctly now.
-Jake
Could that piece of metal be from one of those pierced primers?
I know that when my AR15 pierces a primer due to too-hot loads that a firing pin replacement is in order. The gas from the pierced primer will actually cause a small divot in the end of the firing pin, which then allows subsequent primers to want to bulge out into that divot and pierce, even though the subsequent loads may not be too hot.
I read about this phenomena once and didn't believe it, but I took one of my firing pins to the natural sciences museum next to my work and had them put it under a microscope, which clearly showed that a divot had indeed been created.
