Generally Model 21 chokes were defined by performance rather than the amount of constriction. So if a barrel was to be choked "Improved Cylinder" they would bore the choke out so that the gun shot 50% of its charge into a 30" circle at 40 yards.
As far as I know all makers and manufacturers today would define "Improved Cylinder" by the thousandths of an inch of constrction(say 12 thousandths for IC).
All of the chokes I have experience with shoot tighter than they should. Mod shoots near full, IC shoots mod, etc... But of course I am shooting cartridges with shotcups and plastic wads. Most of the 21s were built in the days of fiber wads.
I onec had a 21 two barrel set. A left barrel was marked "Mod" and was was about 12 thousandths I remember.
Best,
Mike
Last edited by AmarilloMike; 06/13/13 07:28 PM.
