Note that for the 2 3/4" 12ga if both chamber End & bore are on "Nominal" dimensions with 0 tolerance then with the cone angle of 530' the cone length would be 0.358" long. With the SAAMI suggested cone of an even 5 the cone length for same chamber & barrel dimensions would be 0.394".
It is I believe also noteworthy that for dimension E (chamber length) Ithaca uses the nominal length, not the shorter one some companies used. Everyone can, of course, make up their own mind, but personally, I would be leery of using a shell which was longer than the chamber in a cone with a steeper taper (Shorter Cone) than the 5 suggested by SAAMI. Perhaps a max of 1/8" would be OK, but would not want to use a shell a full 1/4" or even 3/16" longer with that short a cone.
I also note this was dated 3-10-39 & that the 28 gauge was still listed as having 2 7/8" chambers at that point. There is also a revision listed which carried the chamber ends for 10 & 12 gauge magnums to a 4 place decimal rather than rounding tp three which was dated 11-6-45 & nothing else seems to have changed during that time.
