I wouldn't hesitate to go for buffalo with a 325 gr, as I have quite a few reports of that weight working fine on them, with lung /heart area hits. The main objective for recommending the 325 gr was to keep recoil less painful, which helps accuacy, which is #1 in taking game cleanly. He can speed it up for the buff hunt, if he feels more secure with higher power, to whatever recoil he can bear, then slow them down for smaller game. However, 1400 fps will do a great job.
If you are opening the checks up, it's quite likely the coldworking, added to the original check forming, is hardening your checks enough that they are springing away from the bullet. Try annealing a few before returning the mold, and if they stay tight, that is your best option. Understand that checks which spring away from the bullet because the metal is hard, will stay on the bullet, but rattle in flight disrupting accuracy. The easiest way to be sure is to measure the drivng bands just forward of the check to see if it is slightly smaller than the checks are. Even a half thousandths inch can let the check turn, so measure close.
Easiest way to anneal a few checks is to lay them around an electric range burner, with burner cold, of coarse. Do it at night, and shut the lights off when you turn the burner on. You''l be able to see a dull red in the dark, at about 850 deg F or so. When you get that, turn the burner off, wait till it cools and remove the checks. Nice and clean and easy and you can probably run 300 at least in one lot. They don't need to be quenched, though quenching will not harden them, if you heat some other way.
