I have a Model 21 in 16 gauge marked 2 3/4 inch on the barrels. It was made, I believe, in 1937. The chambers measure 2 5/8 inches. And yes, it kicks the snot out of me. The short stock does not help. I'm working on getting all of it resolved.

I was told, as Ole Cowboy just stated, it had to do with the roll crimped shells. When they opened, they measured 2 5/8 inches. They were sold as 2 3/4 inch.