Roy Dunlaps book "Gunsmithing" copyrights of 1950 & 1963 show minimum chamber & maximum shell dimensions for the 2 3/4" 12ga.
The maximum shell is 2.76" long "Uncrimped". Maximum length of the loaded shell is 2.530" if roll crimped & 2.410 if fold crimped with a maximum diameter of .797" at end of shell.
Minimum chamber length is 2.6136" to a minimum cone of 5° per side with a minimum diame/ter at juncture of chamber & cone of ."798". He further gives a minimum diameter of .764" at a point in the cone 2.8079" from the breech. While he does not specifically say so I am quite certain these drawings were SAAMI Spec for the era. Note that if a max length shell were fired in a minimum chamber the fired shell would lap in to the cone by .1464" or a bit over 9/64". Note that this is a bit more than using a 2 3/4" shell in a "Nominal" 2˝" chamber (Normally at least (65mm=2 9/16" long) but also note this was sanctioned for "ANY" 2 3/4" shell which met SAAMI Specs.
All of this was well established at least 65 years ago. All these Warnings continue to be Spread because it is undesirable to fire a Heavy loaded, max pressure 2 3/4" shell in a lightweight weight gun having 65mm chamber which was not designed for the heavier load. As noted the British have been loading shells sanctioned for the Nominal 2˝" guns in a slightly longer hull since about the end of WWII.
When WWII ended I was 7 yrs old. I am not certain of Bell's age but I think him younger than me. I do appreciate that is work showed that the change from paper to plastic hulls & card & fiber to plastic wads did not change anything, but to give credit where it is due he did not "Discover" anything, just confirmed what was already well established. Sort of like confirming that a wheel will roll.
