A 1950ish Winchester Model 12 20 gauge that is in great shape for its age. But for decades it has hated ejecting the spent shell and we have not found a gunsmith that can figure out why.
Check the chamber length. Grandpa’s Model 12 in my possession has a 2 5/8″ chamber. Jams on 2 3/4″ shells but cycles 2 1/2″ inch shells like a champ.