Couple of common reasons, both bad news:
Someone lengthened the chamber (or installed a 3" chambered barrel)
Gun is heavily worn and chamber has been blown out so that now 3" shells chamber. I got one with a blown chamber, and had it sleeved by a gunsmith who has now passed on.
Is his ejection port large enough to cleanly pass 3" fired shells? Sometimes "gunsmiths" "fixed" that also if they lengthened the chamber.
Good luck, GJ
A 3” shell will easily drop into a 2 3/4” chamber as most 3” shells are only around 2 5/8” long when crimped. It’s only when fired that it’s too long for a 2 3/4” chamber.
Saved a screenshot. No need to go edit it.