Hello Fess and welcome,

Concerning chamber length, if you somewhat determined that a loaded
2 3/4 shotshell -that measures 2 1/4" when unfired- leaves 1/4 inch
or so of open chamber, then the chambers measure 2 1/2", i.e. 2 1/4
of shell plus 1/4 of free space.

Pictures will help a lot and allow us to see what sounds like a very nice
and interesting double.

JC

"...it is always advisable to perceive clearly our ignorance."ť Charles Darwin