Re: 3 in slugout of 2 3/4 double barrel
A 3 inch shell will fit into a 2 3/4 inch chamber with no problem. A three inch shell is not three inches long till it is fired and a 2 3/4 inch shell is not 2 3/4 inches till it is fired. When a three inch shell is fired in a 2/3/4 inch chamber gun, the crimped end cannot open all the way which becomes an obstruction. It could be dangerous with a rifled slug or even fine shot if the guns action isn't strong. I saw a guy accidentally shoot a 3 in shell in a 2/3/4 in chamber Springfield semi auto Browning clone and all that happened was the gun jammed because the shell couldn't be ejected. I wouldn't recommend doing it.
When you are up to your butt in alligators, it is hard to remember your intent was to drain the swamp. Stay focused!