The easiest choice is to buy hevi shot, but it does get expensive.
If you can get your hands on a variety of steel loads, take them out and test them. Take a large sheet of paper (I can't think of the paper type, but the kind used to draw on for meetings and such) and draw a large circle.
Shoot the variety of loads at 35 and 50 yards and keep track of the pellet counts inside the circle.
What load works best depends on the gun/barrel itself.
If you can get your hands on a variety of steel loads, take them out and test them. Take a large sheet of paper (I can't think of the paper type, but the kind used to draw on for meetings and such) and draw a large circle.
Shoot the variety of loads at 35 and 50 yards and keep track of the pellet counts inside the circle.
What load works best depends on the gun/barrel itself.