I have a copy of an "Ithaca Gun Co., Inc. Ithaca, N. Y." drawing number "T-113" which is titled " Standard Bore, Chamber & Choke For Shotguns". It was drawn by "NFS" on 3-10-39. With one minor revision on 11-6-45.
For both 12 and 12 Mag. the:
Bore diameter is listed as 0.729;
Full choke is 0.693 dia and 2 1/2" long;
Imp Mod is 0.702 dia and 1 7/8 " long;
Mod is 0.711 and 1 1/4" long;
IC and Skeet is 0.720 dia 5/8" long and
Cyl is 0.729 and 0" long.
The length of the choke is where the choke begins at bore diameter to the muzzle end. For example, full choke starts back 2 1/2" from the muzzle at 0.729 bore diameter and ends at 0.693 at the muzzle. No clue about taper or rate of change.
As pointed out, this is a 1939 dated drawing so anything prior to this could be different.
Don't know if this helps but there it is.
Bill Jolliff
