What we also have to remember here is that it was quite common--especially with target shooters of that era--to use shells nominally somewhat longer than the chambers in which they were fired. (So documented in a 2 part American Rifleman article from the 30's.) The paper case mouths, opening into the forcing cone, gave some protection to the shot on its initial contact with the bore. Result: fewer flyers, better patterns.
My Lefever barrels (1901 gun) are chambered 2 5/8". The second set, about 15,000 later (but renumbered to the gun), are 2 3/4" and do not look to have been modified.
I don't know at what point Hunter Arms went to 2 3/4" as standard for the 12ga. (2 9/16" was still their standard for the 16 as late as 1938.) But American gunmakers must've made quite a few short-chambered 12's, because as late as 1940, the short shells offered by American ammo makers outnumbered 2 3/4" 12's by about 2 to 1. My guess would be that a target gun, unless the buyer specified otherwise (and some did, so they could shoot long shells in short chambers), would have been more likely to have 2 3/4" chambers than a field gun.
