I'm not quite ready to declare that I've found a cure for the DPMS Prairie Panther, but I'm close. The heavier bullets (Hornady 75 gr. BTHP match) and additional break-in seems to be doing the trick. Rifle averaged an inch or a bit under, with best group at .60 inch. Had two bullets in the same hole twice, indicating the rifle may be capable of even better accuracy than the average for groups, but the trigger is really testing my patience. It's a heavy two-stage, and I'm not very fond of it. I may replace it with something better in the near future. No issues with shifting point of impact this time. To test, I unhinged the upper once and closed it, without cleaning, about halfway through the session. No issues there, so I fired a few more groups and then unhinged the upper and cleaned the barrel. Again, point of impact held. Rifle seems to be settling in now. I have a total of about 100 rounds through it, so I'm thinking the earlier issues were more about break-in and the rifle's preference for heavier bullets.
