Well...like mentioned above, seems that I didn't quite fully-seat the primers to start with. I loaded a bunch of 115gr RN plated bullets prior to the XTPs - same recipe, same Fed 100 primers. I always stand up each cartridge on the bench when it comes off the press and I got zero wobble with any of those. I know the XTPs need to be seated deeper, but the primer was flush in the primer pocket with the platted bullets so I figured everything was fine.
I prepped some new cases and turned out the seater plug of my hand primer one big turn, then loaded some more XTPs. Everything was fine after that. And to my eye, it almost looks like that primer is a hair beyond flush in the primer pocket.
I pulled the previously-loaded bullets, resized and re-expanded, then ran them through the hand primer with the new setting and sure 'nuff - those loaded cartridges came out just fine too. Now - let's just hope everything goes bang!
Thank you all for the help!
