Found this 1949 Winchester 21 Choke Sheet. Appears to be copyrighted at http://bluepike.tripod.com/Model21/Model21Tables.html - maybe the web site contents are copyrighted.

Some of their values show less constriction for the choke than I am used to seeing.

WINCHESTER Model 21
Bore and Choke Specifications

GAUGE
BORE DIAMETER CHOKE % in 30" Circle @ 40 yards*

..........CHOKE DIAMETER CONSTRICTION

12 .730" + .005"
Full
65% - 75% .694" - .699" .031" - .036"
Imp. Mod.
55% - 65% .712" - .715" .015" - .018"
Mod.
45% - 55% .715" - .720" .010" - .015"
Imp. Cyl.
35% - 45% .724" - .726" .004" - .006"

16 .670" + .005"
Full
65% - 75% .640" - .645" .025" - .035"
Imp. Mod.
55% - 65% .652" - .655" .015" - .018"
Mod.
45% - 55% .655" - .660" .010" - .015"
Imp. Cyl.
35% - 45% .664" - .666" .004" - .006"

20 .615" + .005"
Full
65% - 75% .590" - .595" .020" - .030"
Imp. Mod.
55% - 65% .600" - .603" .012" - .015"
Mod.
45% - 55% .603" - .608" .007" - .012"
Imp. Cyl.
35% - 45% .609" - .611" .004" - .006"


* Figures from "Winchester Gun Salesman's Hand Book" ©1949

©2010, M.C. Manges

Last edited by J. Hall; 12/12/11 09:45 AM.