Folks shooting the .300 Win Mag with max loads in long range matches have got 15 to 20 reloads per case. They full length size using dies with necks a couple thousandths smaller than a loaded round's neck diameter. And don't set the fired case shoulder back more than 2 thousandths. A second body die's often used that sized the case all the way to the belt; normal dies don't. Larry Willis makes collet dies that do the same thing these days www.larrywillis.com
I've got that many reloads per .30-.338 case.
Best powders used in competition for accuracy with 190, 200 and 220 grain bullets was/is IMR4350 or some other extruded powder of the same speed. Slower powders may well shoot bullets out faster, but they won't land on target very close together.