Tuesday, February 22, 2011

Shotguns: Chokes and Choke Boring

When a shotgun fires multiple pellets, they spread out in a cloud of pellets upon leaving the barrel. This is known as the shotgun pattern or shotgun shot spread. We've already studied how to determine the shotgun pattern earlier and the reader is advised to refresh their memory from the earlier article.

Now the key is that the largest number of pellets must penetrate in a 30 inch diameter circle, such that if a bird silhouette was to be placed on the circle, that the silhouette cannot be placed anywhere where at least 3 pellets are not going through it.

In order to increase the number of pellets within the 30 inch circle, a choke is often employed. Chokes may be built into the barrel, as part of the manufacturing process, or the end of the barrel may be threaded and the user can screw on a removable choke to the end of the muzzle as needed. This way, the user may be able to use different chokes depending on the number and diameter of the pellets used.

Since removable chokes are more modern, we will now study the history of choked barrels (i.e.) barrels manufactured with a built in choke.

The basic principles of choke-boring seem to have been invented by Spaniards, as we find the first mention of improving shooting patterns by various boring methods in Spanish books. M. de Marolles in his book, La Chasse au Fusil, states that some gunmakers in his time maintained that, in order to throw shot more closely, the barrel diameter should be narrower in the middle than on the breech or the muzzle end; while others insisted that the barrel must gradually contract from breech to muzzle. He goes on to describe methods to achieve these results, as were in vogue during his time. J.W. Long, an American author, in his book, American Wild Fowl Shooting, claims that choke-boring was an American invention and attributes the discovery to one Jeremiah Smith of Smithfield, Rhode Island, who was making choke-bored barrels, as early as 1827. The first known patent was granted to an American gunsmith, one Mr. Roper, on April 10th 1866, who preceded another claimant, an English gunmaker, Mr. Pape, by just six weeks.

While these early inventions were by American gunsmiths, they had not fully understood choke boring and therefore, a lot of their guns would lead, shoot irregular patterns and not shoot straight. It was left to an English manufacturer, W.W. Greener, to invent a method of choke boring that became the most widely used method in the later part of the 19th century. It was because of the popularity of the Greener method of boring that some authorities falsely give W.W. Greener the credit for inventing choke boring, though he himself never claimed to invent it.

W.W. Greener was a well-known gunmaker in Birmingham in the 19th century (the firm is still around today). His first intimation of a choke formation was from a customer's letter in early 1874. This customer had ordered a custom gun and in his special instructions to Greener, he described a choked barrel, though he did not specify its size or shape, or how it was to be obtained. However, W.W. Greener was intrigued enough to conduct many experiments to determine how to make the best profile and size of the choke for any given bore diameter. He also invented new tooling to make this boring possible. After many months of experimenting, he figured out how to make appropriate choke profiles for any bore of shotgun.

The Greener choke consists of leaving the barrel mostly cylindrical, but creating a constriction in the barrel towards the muzzle end of the barrel, as can be seen in the figure above. Before this method was invented, most people would either make the breech end of the barrel of a slightly larger diameter for up to 10 inches of barrel length from the breech, or they would bore the middle of the barrel to a smaller diameter and make the breech and muzzle of a larger diameter, or they would simply leave the barrel as a true cylinder (no choke).

On December 5th 1874, Mr. J.H. Walsh, the Editor of Field magazine,  mentioned the Greener choke in an article, that read:
"We have not ourselves tested these guns, but Mr. W.W. Greener is now prepared to execute orders for 12-bores warranted to average 210 pellets of No. 6 shot in a 30-in. circle, with three drachms of powder, the weight of the gun being 7.25 lb. With larger bores and heavier charges, he states that an average pattern of 240 will be gained. As we have always found Mr. W.W, Greener's statements of what his guns would do borne out by our experience, we are fully prepared to accept those now made".

The article created a sensation because the very best 12-bore shotgun in the London public gun trial of 1866 could only generate an average pattern of 127. The very next issue of Field magazine contained an ad from W.W. Greener guaranteeing a pattern of 210 on his 12-bore guns. There was also a letter to the Editor in the next issue, from a reader of the magazine, confirming that his latest purchase from W.W. Greener did indeed meet this claim and more. Naturally, such statements created a huge controversy among gun manufacturers and readers, and the Editor of Field magazine was compelled to send a Special Commissioner to witness and verify the shooting of  Greener guns. The Special Commissioner not only verified the claims, he actually got an average pattern close to 220 during his testing! After that, several other manufacturers claimed to be in possession of the same method of boring as W.W. Greener and therefore, the proprietors of Field magazine decided to conduct a public trial, the London Gun Trial of 1875, to verify various manufacturer claims. Greener-made choke bores won overwhelmingly in this trial, as well as the London trials of 1877 and 1879 and the Chicago trials of 1879 and led to the fame of his company spreading.

The Greener method of choke-boring was later adopted by other manufacturers and became the dominant form of choke boring. Modern chokes today are usually screwed on to the muzzle end of the barrel and slightly change the diameter of the muzzle in much the same way.

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