I've heard it told that people often misjudge a 2.5" chamber because they load a standard 2.75" shell in the chamber and the action closes on it.
I've often wondered if you can therefore make a rough estimation of a standard 2.75" chamber by seeing if the action will close on a 3" shell?
If the gun isn't properly marked as to chamber length, it could also have a 2-1/2"(2-7/16") chamber, and a 3" shell chamering isn't a relible indicator of chamber length.
Fahgetabout using the EASY
button, and measure the chamber(s).