Slightly short chamber, fixed by AA CS

[Jakal]

Yes jamming the bullet into the lands can cause over pressure. But even with longer throated barrels, factory loads will still sometimes run hot and show flattened to near flattened primers. Seat your bullet a little deeper and about .5 (1/2 grain of less powder from your minimum load and see if you still have the issue, 3 rounds should do it.

The factory Hornadys are loaded short, kind of a known fact. So if you are jamming with factory sst, then i would take the advice of a finishing reamer.