Choke Chart
Choke is a carefully measured constriction of the bore of a shotgun at the muzzle, designed to control the spread of the shot as it leaves the barrel.
Hallowell & Co.'s descriptions of choke borings are determined by measuring with a bore micrometer, irrespective of any markings on the barrels. The internal diameter is measured four inches from the muzzle and again just at the muzzle. Subtracting, gives the amount of constriction in thousandths of an inch.
In our descriptions of each gun, chokes are listed in the order of the normal sequence of firing and abbreviated as under the following American Designations:
| 12 Gauge | 16 Gauge | 20 Gauge | 28 Gauge | .410" Bore | American | English | Browning | Italian | Spanish | Perazzi | Percentage | |||||||||||
| Constriction | Constriction | Constriction | Constriction | Constriction | Designation | Designation | Codes | Codes | Codes | Codes | Pattern | |||||||||||
| .000" -.001" | .000" - .001" | .000" - .001" | .000" | .000" | cylinder | true cylinder | *** | C**** | ***** | 0 | 40% | |||||||||||
| .002" - .006" | .002" - .005" | .002" - .004" | .001" - .004" | .001" - .002" | skeet | impr cylinder | **$ | S | 1 | 45% | ||||||||||||
| .007" -.013" | .006" - .012" | .005" - .011" | .005" - .007" | .003" - .006" | impr cyl | quarter | **- | **** | **** | 3 | 55% | |||||||||||
| .014" -.023" | .013" - .021" | .012" - .019" | .008" - .014" | .007 - .012" | modified | half | ** | *** | *** | 4 | 60% | |||||||||||
| .024" -.031" | .022" - .028" | .020" - .026" | .015" - .020" | .013" - .017 " | impr mod | three quarter | *- | ** | ** | 6 | 65% | |||||||||||
| .032" -.040" | .029" - .037" | .027" - .033" | .021" - .027" | .018" - .021" | full | full | * | * | * | 9 | 70% | |||||||||||
| .040"+ | .038"+ | .034"+ | .028"+ | .022"+ | extra full | 10 | 75% |
The dimensions used in the above table are rules of thumb. Bore measurements by micrometer are useful to predict the pattern thrown by a shotgun barrel, but they remain merely a prediction. The only way to determine the actual pattern thrown by a shotgun barrel is to shoot it, by convention at 40 yards, count the percentage of pellets falling within a 30" circle placed around the visual center of the pattern (as in the right-hand column of the above chart), then do it a few more times and take an average.
For more than you ever wanted to know about choke boring, see: The Mysteries of Shotgun Patterns, George G. Oberfell & Charles E. Thompson, Division of Engineering Research, Oklahoma State University Press, Stillwater, 1957
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