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.
i think the remington heavy shot works best in any conditions. but it can definitely get expensive so another good load is the winchester dry/lok
also try shooting #1 shot ive heard it can do the job ive only seen them in federal though. make sure on the #1 shot you get it inn steel.
You will just have to figure out which load patterns the best out of your gun. Most mossbergs have a slightly larger bore size in the barrel so they tend to patter larger shots (T's BBB's and BB's) better than other guns. I personally like #1's. You get a few more pellets but they are still big enough to knock em' down. But latter in the season if your shooting 3" steel I would highly recomend Hevi-Shot or at least using BB's and BBB's.
But like Chris said, go get some butcher paper ( I think that's what he meant) and pattern your gun. That is the best way to find out.
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