RE: buckshot
as the others said,you need to pattern your gun with different size and name brand shells.
my BPS 3"will shoot Winchester #4 buck better than any others.
The best way to tell is get a sheet of plywood,(4'x8') and put a paper plate in the middle.
step back say 30 to 40 yards and shoot each different one,if you find one that can put several pellets in the plate then step back another 5 yards at a time.this will tell you if you have a killing load.it only takes a few pellets if they are in the plate.