So, the gun was originally chambered for 2 3/4 inch shells. In those days the heaviest "factory" 20-gauge loads were the 2 1/2 drams of bulk smokeless powder pushing 7/8 ounce of shot. That load could be had in 2 3/4, 2 7/8 or 3-inch cases, the longer two just having more/better wadding.