I would just use some 00 Buckshot, 2 3/4" 9 pellet. Classic stopping round. Low recoil versions are good also. I wouldnt use slugs, they penetrate too much, and birdshot is for birds. Ive use the 3" magnum 15 pellet loads but the recoil is substantial, and they dont seem to eject well out of the mossbergs.
remy 1100 here. Great if you like an auto loader. I need to go out and shoot soon.