If it has a plug, which most likely it does, it has been shaved down to allow a extra shell if 2 1/2” I expect. Easy to fix. Knew fellows who used short shells in a gun which had a plug for 2 3/4” shells. Those mini shells would allow three up the magazine instead of just two 2 3/4” shells. Game warden would try shoving a third “regular” shell in the gun and it would not go. Fad did not last long when one fellow was seen shooting a fourth shell at a dove without reloading. Got his ticket from the game warden and a lot of ribbing for needing four shells to miss a single bird. Heck, most could miss any bird with only one or two shells. Even game wardens can count to three, all the way up to 15 these days. When I as young they only had to count to 12. Inflation.
