Assuming you are using standard loads, consider that the old guns were chambered to work with thick-walled (approx. 0.025") paper hulls. Today's premium plastic hulls like the AA and STS have only a 0.005" thickness at the mouth. That buys a lot of breathing room, and you can use the longer plastic hull without impinging on the throat like a paper hull would. Use a bore gauge to check it out.