What really works--and I mean accurately--is Lyman's 20 gauge, sabot slug. Dropped from the mould it has a diameter of 0.576", weighs 350 grs. and looks exactly like a big diablo airgun pellet with a hollow base.
Use 60-70 grs of 2 or 3F, a 1/8" nitro wad over the powder followed by a greased...