If someone fired a hot handload in it, the .223 primer may be crattered back into the firing pin hole. That could act more or less as an additional lug preventing the action from opening. You could probably tell if there is a fired cartridge in the chamber by putting a cleaning rod down the muzzle end of the barrel. I guess the end of a cleaning rod would be about 1/8th inch short of touching the breech face. Also, maybe the problem is a crattered primer in a cartridge case the shotgun barrel.
