My second shotgun ever owned was a 2 3/4” chambered 1100. In my youth, I never thought twice about shooting 3” shells in the gun. Other than an occasional ejection issue due to the added length of the expended 3” shell barely being able to clear the port. I never had an issue and shot probably thousands of rounds through the gun. I realize your issue is regarding an old gun and pressures are the general concern, but shell length in a shotgun, in my very limited experience, should not be an issue in a single shot.
