I measured my 16ga L.C. Smith as per your reccomendations, by taking a 3x5 index card and rolling it up pushing it down until it stopped. I did it twice in each side and came up with 2-3/4" both times. I was told by the last owner to shoot 2-1/2" shells in it, but by measuring 2-3/4" can I shoot 2-3/4" shells ?