A lot of people have gotten away with shooting current factory 2 3/4" shells in old American 12's for a couple reasons. First, most 2 3/4" 12ga shells are really only 2 5/8" long when fired, or just a little over, so you're not opening the case mouth into the forcing cone. Second, American 12's--for the most part--tended to be pretty stout.
If you change that scenario to a 20ga, where the old standard chamber length was 2 1/2", and if you try modern factory ammo in some of the very lightest old American doubles, then you can have an issue. Some of the really light Ithaca Flues 20's, in particular, have ended up with cracked frames.
Also, the smaller the gauge, the higher the pressure. That may be another reason why it's less common to see "catastrophic failures" in old 12's.
Last edited by L. Brown; 05/16/10 06:38 PM.