Sport with Gun and Rod in American Woods and Waters by Alfred Marshall Mayer,
1883https://books.google.com/books?id=IJcCAAAAYAAJ&pg=PA781&vq The choke-bore now almost universally adopted by gunmakers is as follows: Taking a twelve-gauge gun as an example, the construction of the bore from the front of the breech-chamber to within one and a half inch of the muzzle amounts to about 1/100 of an inch. At one and a half inch from the muzzle begins a sharp contraction which, in the length of one inch, equals 25/1000 of an inch. The last half inch of the bore is a true cylinder.
.010 + .025 = .035" constriction = 5% for "choke bore"
.410" X .05 = .0205"
The Gun and Its Development by W.W. Greener, 1907
http://books.google.com/books?id=3HMCAAAAYAAJ&pg=PA416 The various degrees of choking may be classified as:
Full-choke, which, with a 12-gauge gun, standard load, distance and conditions will make an average pattern of 215 pellets
The half-choke (same conditions) ... 185
The quarter-choke... 160
The improved cylinder... 140
The old cylinder…115
Any better average shooting than 215 may be termed an extra full-choke; the improved cylinder is a barrel very slightly choked.
Unfortunately, Greener doesn't define his "standard load". If it was 1 1/8 oz. No. 7 Tatham chilled shot (291 per oz.), then 1 1/8 oz. = 215/327 = 66% for Full-choke
On p. 354 he states 1 1/8 oz. No. 6 is 305 pellets. 215/305 =
70%https://books.google.com/books?id=3HMCAAAAYAAJ&pg=PA354
