Hi there, Be slightly careful with chamber length as 3 1/4 was generally the longest chamber in US made guns, though some may have been longer.. British chamberings went up to 4 1/4 from memory I used Winchester 3 1/4 magnum paper hulls from a cement works. I had to make up a swage to tap the hulls into to reduce the second brass thickness back to parallel so they would chamber. It soon worked out that a light rub inside and out the cartridge mouth with melted paraffin wax extended case life significantly on the paper cases. The load I used was the standard ICI - Eley-Kynoch recommended load for 3 1/4 cases of 158 gn FFG and 2 ozs shot as the gun - an American Davenport, has no proof marks, I didnt push it to 2 1/2 ozs shot. Being a single barrel I didnt need to wory too much about crimping so just used hot wax on the over shot card but this will not work in your double so your on your own there I am afraid.