Buckshot, when used with the correct choke, will give you about 75 yds range. As long as you can keep two pellets on on 11" paper plate, you are good to go. Since you don't have a rifled barrel, you will only get about 75 yds out of a slug. I am a slug fan, but buckshot has more knockdown power if you can get multiple pellets to land on target.

good luck!