Here's what W.W. had to say in
The Gun and Its Development 8th Edition 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.
