Re chamber length, pressure, etc:
I saved a copy of a post from this BB going back to 1998. In that post, the author quoted from an article by LTC Calvin Goddard in the May-June 1934 issue of "Army Ordnance". Goddard was quoting from the SAAMI proof tables then in use. Per those tables, there were different proof and service pressure standards for 2 5/8" 12ga vs 2 3/4" 12ga. Mean service working limit for the short shells: 9,500 psi. For 2 3/4" shells: 10,500 psi.
We know from the tests on which Sherman Bell reported in Double Gun Journal that there is a pressure increase when firing 2 3/4" shells in a 2 1/2" chamber vs 2 3/4" chamber. (Average increase of 738 psi, maximum of 1216 psi, from the data he reported.) While there may well be less than that for a gun with 2 5/8" chambers vs 2 3/4", the nominal chamber for the short 20's was 2 1/2", and 2 9/16" for the 16's--where it may be more of an issue. But you're already starting "in the hole" simply because your 2 3/4" shell has a higher SAAMI service pressure than the shorter shells. To which you then must add whatever increase is due to longer shell/shorter chamber. Looks to me like it could be substantial, especially in 20's--and more of an issue in 16's than 12's.
