I had a similar problem loading 300 blackout – was actually slightly crushing the necks and causing issues when plunking in the barrel. Took me forever to figure out. Grabbed a Lyman M die and just ever so slightly belled the case mouth to just allow it to sit nicely on the case. Biggest issue is to make sure you use a crimp die after seating to push that bell back in, or it won\’t chamber correctly.