I put 1 inch pipe in the one I modded, it's pretty close to that size.
If you stick the pipe 1 1/2 to 2 inches into the muffler it's not too loud. If the pipe is really short it will be quite loud.
It's a barrel STUB. The choke was already cut off around 24" and I cut it further to 20" with extended mag. I don't cut up shotguns for chainsaw mufflers.
I have a whole pile of sawed off shotguns. That's my business. I also have some leftover barrel pieces. Is a 12 ga barrel of suitable diameter for an unported 372XP?