This is from my files, I saved it from a earlier post. Reprinted without permission.
Tyler
Here is a copy of something I put together in response to a similar question on another BB.
The traditional way of designating chokes (before the advent of interchangeable choke tubes) was by the patterns they produced. Don Zutz says (in a chapter of Lyman's 4th Ed.) that all pattern percentages are measured in a 30" circle at 40 yards, except for the .410 bore, which is measured with a 20" circle at 20 yards. He cites the following percentages as "industry standards" without specifically naming SAAMI.
Cyl 40%, Skt 45%, IC (British 1/4 choke) 50%, Skt 2 (often called light Mod, Brit 3/8) 55%, Mod (Brit 1/2) 60%, IMod (Brit 3/4) 65%, Full (British "choke") 70%, Ex F 75%.
Every combination of gun, choke tube, and shell may produce a different percentage, so many manufacturers of choke tubes have adopted constriction designations instead of percentage designations. I really don't know if the manufactureres are consistent (I suspect they are not) but the famous shotgun gunsmith Ralph Walker listed the following constrictions in The Gun Digest Book of Shotgun Gunsmithing, 1983.
12 GAUGE: Skt .005, IC .009, Skt2 .012, Mod .019, IMod .025, F .035. These are close, but not quite the same as Briley's specs, as printed on the back of a business card (about 1998 vintage) from Stephen Power, Sales Manager, which are as follows: Sk .005, IC .010, LM .015, Mod .020, IMod .025, LFull .030, F .035, XF .040. It looks like Briley has simply rounded off the numbers for the sake of convenience.
Continuing now with Walker's data:
16 GAUGE Sk .004, IC .007, Sk2 .010, Mod .015, IMod .020, F .028
20 GAUGE Sk .004, IC .007, Sk2 .009, Mod .014, IMod .019, F .025
28 GAUGE Sk .003, IC .005, Sk2 .007, Mod .012, IMod .016, F .022
.410 Bore IC .004, Mod .008, F .017
Now I am begining to wonder why I went to the trouble of looking up all this data and typing it. I guess it is because I am an engineer by birth as well as by education, and engineers just love data. I have been suffering from data withdrawal since I retired. I hope someone finds it usefull. You might want to copy this response and save it as an MSWord or WordPerfect document, for future reference. I will, because I don't want to have to get my data fix this way again!
Jim Cassada (No, I am not Jim Casada, the writer/editor!)