Agree with Ed. Sounds like a sear issue. I'd leave that one to a gunsmith. As for chamber length, if the gun dates back to pre-WW1 years and especially if it's either a 16ga or a 20ga, then it's very likely that the chambers are shorter than 2 3/4". Better chance that it's 2 3/4" if it's a 12. But even then, it might be short chambered. Note that I'm speaking of ORIGINAL chamber length. Quite a few guns that left the factory with short chambers later got 2 3/4" chambers courtesy of a gunsmith.