1) Wet tumble to clean inside of brass and primer pocket for case capacity consistency and to skip primer pocket cleaning.
2) Anneal .... you will hear different things but I anneal every time. Who's right? I don't know.
3) Full size with .002 bump. Watch youtube Eric Cortina video if needed...
I would check sami dimensions vs the ammo. I'm guessing this is 100% an ammo quality control problem but there is an outside chance its the rifle. You'll know after checking sami.
Get a dope book and notate everything so next time you'll have first rnd hits. Get an app or something that will get you close. This distance isn't really a big challenge for 300 prc. Have fun!
Few bullets like being seated on the lands. Try .020 off 3 to 5 shots each at .003 increments. Look for 2 groups close together. Then do your ladder test again. It will change. Then you can forget about the above and just shoot dimes.
Start at max length allowed. 5 shots, then increments of .003 deeper. 5 shots each. Look for 2 groups next to each other. Then do 1 more ladder test to make sure.