Jeremy: here's an easy way to measure the chambers yourself
http://www.lcsmith.org/faq/chamlgth.html
I once had a 1928 20g Trojan with chambers close to 2 3/8", and recoil in that light gun was
brutal with 7/8 oz 2 3/4" loads.
Several 2 1/2
inch 20g shotshell sources are listed here
http://www.lcsmith.org/faq/loadsuse.html