I like your thinking. There were at least 2 10 bore case sizes (diameter not length) in the Holland 10's. A .775 diameter "paper case" gun and an .805 diameter "thin wall" brass gun. A 2 7/8" .805 case accommodates 8 drams of black and an elongated bullet (.968 in length) nicely. A 2 5/8" .775 case accommodates the same bullet and only 6 drams of black powder. A 3 1/4" .775 case would probably be an 8 dram gun or more but shoot the gun and let it tell you.