But you have an ASP modded breech. That does the same job as an insert kit. If you underbore with an insert kit, you don't get rollouts. This acts as a detent system as well as allowing for the best efficiency (no air is lost around the ball) and accuracy (bore to ball size is the best way to achieve accuracy). Though it doesn't stop double feeds like an ASP modded breech will.
Yes, the TPC could also be culprit. Definitely check into that as well, in addition to everything else Michael pointed out.