WARNING !!! A 3" shell will drop into a 2 1/2" chamber. The problem is that when a 3" shell fires, the case opens up and becomes longer. it is the extra length that causes high pressure. I agree, it may be converted to 3". I would much prefer a 3". Big time. I would want to know. A snap cap is not a test.
There are a couple tricks or methods to determine. Starting with a narrow steel ruler and super sharp eyeball. You can turn a wood/metal rod to exact diameter of the ammo and measure how deep down it goes. Another WARNING, the transition from chamber to barrel is most likely a ramp and not a sharp edge. Hard to get an exact read on where the chamber ends. Obviously, you can do a chamber cast. if you had the materials I am sure you would have done that already.
I have an H&R folder in 410. These old small bore shotguns are much sweater handling than some of the more recent guns. In the old days, they had gauge specific frames. Later on it was one frame for 410 up to 12. Made for a nice 12, but; a stupid heavy 410.
