All 12 gauge Model 12s were made with 2 3/4" chambers. Smaller gauge (16, 20) were short chambered. Simmons used to do the 2 3/4" chamber work and may still do it. Might want to talk to them about it.
1916XE- If your stock has not been altered, pull the butt plate. You may find TOURN. stamped into the wood. That will indicate that you have a Tournament Special. From your description of your gun it sounds like that is what you have. Is yours a straight stocked gun?