Check the chokes as they may have already been opened. For as much as you intend to shoot it just buy a couple flats of short RST shells and no worries. If you really want to shoot full power 2 3/4 in. shells have the wall thickness at the chamber, checked and maybe lengthen the forcing cones. Likely in it's past. it was fed long shells as that was all there was after the ammo companies stopped making short ones.