It's Not Rocket Scienceby John McGonigle
Shotgun chokes and patterns can be as simple or complex as one wishes to make it. I prefer the former.
In his 1965, The Shotgun Book, Jack O’Connor related that his predecessor at "Outdoor Life", Captain Charles Askins, attributed the discovery of choke to Fred Kemble. Apparently, O’Connor misspelled the choke inventor’s name because Askins’ book, Modern Shotguns and Loads, 1929, lists the name as Fred Kimble, as do several other books in my library.
Kimble was a market gunner from Illinois who, because of necessity, needed maximum effectiveness in his shotgun. He tried various adaptations to his muzzleloader before stumbling on the previously unknown technique of adding effective "choke". He put too much choke in his early attempts and found that his gun shot worse than before. Experimenting further, he came upon the solution and created a shotgun that would pattern, or spread its pellet load, more effectively than anything previously available.
In 1929 Askins reported, " Mr. Kimble had accidentally learned all that is known of choke boring to this day." Even though Kimble did his work in the late 1860’s, Askins was pretty close to being right, even today.
Adams and Braden, two American Anglophiles, credit William Rochester Pape, an English gun maker, with patenting the "choke bored" barrel in 1866, in their instructive book, Lock, Stock & Barrel. O’Connor and others also credit Pape with the invention of choke.
It is certain that others both in America and England were working on the concept of choke at the same time as the gentlemen mentioned.
Choke is the constriction at the muzzle of the barrel, in thousandths of an inch difference from the diameter of the bore. The word "difference" is where people get confused when discussing choke. To determine choke, one measures the diameter of the bore and the diameter of the choke area at the muzzle; the difference between the two measurements determines "choke".
Industry standards govern shotgun bore diameters at: 10 gauge, .775 inch; 12 gauge, .730 inch; 16 gauge, .670 inch; 20 gauge, .615 inch; 28 gauge, .550 inch. A .410 shotgun does not have a gauge designation; it has an actual muzzle bore of .410 inches.
Choke constrictions have been standardized in inches as follows: cylinder, .000"; improved cylinder, .010"; modified, .020"; improved modified, .030"; full, .040". In other words, differences between bore and choke area measuring as above, provide the listed amount of choke.
An older choke chart shows: full choke barrels are designed to put 65 to 75 percent of their pattern in a 30 inch circle at 40 yards; improved modified, 55 to 65 percent at 40 yards; modified, 45 to 55 percent at 40 yards; improved cylinder, 35 to 45 percent at 40 yards. Modern shotguns using plastic shot cups will nearly always pattern to the high end of the chart, sometimes a bit higher.