The chamber drawings I mentioned were from Greener, 1910 (9th Edition)
From Burrard:
Vol II page91;
"The ordinary 12-bore cartridge is ""Nominally"" (emphysis mine) 2½ inches long, although to be exact the unfired (I think he is here referring to a new unloaded case) case is 2 9/16 (65mm??) inches in length."
"The standard American case was always slightly longer than the nominal 2½ inches and was actually 2 5/8 inches in length.
Vol II, page 154;
"In 1938 I tested some of the first lot of British fully crimped paper-tubed cartridges which were issued. These, as has already been explained, were loaded in 2 9/16 inch cases. I fired a series of ten shots for pressure and obtained very even and perfectly normal results. Two months later I tested another lot which were loaded in ""2 3/4"" inch paper-tube cases. the shot charge in both lots was 1 1/16 ounce and the powder charges were similar. I fired twenty-five shots for pressure and again obtained a very even series with ""NO"" higher pressures than those given by the first lot. Since 1946 I have tested many more lots and these results have ""PROVED"" quite definitely that ""For All Practical Purposes"" any increase in pressure due to the ""LONGER"" case really does not exist ""Provided"" the correct powder and shot charges for a nominal 2½ inch cartridge are used. It is true that all American fully crimped cartridges loaded with but 1 1/8 ounce of shot develop pressures which are considerably higher than those given by the corresponding British cartridges, but this is due to the relatively higher powder charge adopted by the Americans rather than to their use of the 2 3/4 inch case."
This was all virtually settled well over a half century ago, what part do we fail to understand??
The "Load" should be suitable for the gun in question.
The "Loaded" shell should have clearance & not enter the cone.
""IF"" an exceedingly old gun with very short & abrupt cone results in tearing of the crimp, use shorter shells.
Otherwise concern yourself with pointing the gun.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra