Older SAAMI Specs for the 12 gauge 2 3/4" chmaber specified a minimum length of 2.614" length with a minimum diameter of .798" to a minimum cone of 5 per side. To a standard size bore this will give a cone length of about .400". At 2 3/4" this also gives a diameter in the cone of .774" or .024" smaller than the chamber end. These specs were in effect well beyond the introduction of the 1 oz 12ga "Short Magnum" load. This load should have of course been used only in those guns of later manufacture designed & proofed for the heavier loaded shells, but the problem with the older guns was not as a rule in the chamber dimensions themselves, but the overall strength of the gun.
"ANY" gun with its chamber ending in an abrupt step or cone of extremely short design should of course never be fired with a shell longer than its chamber. Certainly not one in which the crimp upon opening will extend up into the bore itself. Also no gun should be fired with a shell long enough in its "Loaded" length to actually enter the cone. As to the gun with 65 mm chambers which will not properly close with a 67/67.5 mm shell, you have some problem other that the length of the shell. As Larry correctly pointed out the loaded length of that shell will not extend to the end of the chamber, it will only do so after firing.
