A choke is a tapered constriction of a firearm barrel at its muzzle end. Chokes are most commonly seen on shotguns, but are also used on some rifles, pistols, or even airguns.^[1] Notably, some .22 LR match rifles have a constricted bore diameter near the muzzle.^{[citation needed]}

Chokes are almost always used with modern hunting and target shotguns to improve performance. Its purpose is to shape the spread of the shot "cloud" or "string" to gain better range and accuracy, and to deliver the optimum pattern of pellet density, for the particular target, depending on its size, range, aspect and whether it is traveling towards, across or away from the shooter.^[2] Chokes are implemented as either screw-in chokes, selected for particular applications, or as fixed, permanent chokes, integral to the shotgun barrel.

Chokes may be formed at the time of manufacture either as part of the barrel, by squeezing the end of the bore down over a mandrel, or by threading the barrel and screwing in an interchangeable choke tube. Chokes may also be formed even after a barrel is manufactured by increasing the diameter of the bore inside a barrel, creating what is called a "jug choke", or by installing screw-in chokes within a barrel. However implemented, a choke typically consists of a conical section that smoothly tapers from the bore diameter down to the choke diameter, followed by a cylindrical section of the choke diameter. Briley Manufacturing, one maker of interchangeable shotgun chokes, uses a conical portion about 3 times the bore diameter in length, so that the shot is gradually squeezed down with minimal deformation. The cylindrical section is shorter and usually between 15 and 19 mm (0.6 to 0.75 inches) in diameter.

A choke is designed to alter or shape the distribution of the shot as it leaves the firearm. For shooting most game birds and clay pigeons, a desirable pattern is one that is as large as possible while being dense enough to ensure multiple hits on the target, at a particular range. The choke should be tailored to the range and size of the targets. Shotguns intended for defensive use often have cylinder or improved cylinder chokes for the widest shot pattern at typically short defensive ranges; "cylinder barrels" have no constriction. A skeet shooter shooting at close crossing targets might use 0.13 mm (0.005 in) of constriction to produce a 75 cm (30 in) diameter pattern at a distance of 20 m (22 yd). A trap shooter shooting at distant targets traveling away from the gun might use 0.75 mm (0.030 in) of constriction to produce a 75 cm (30 in) diameter pattern at 35 m (38 yd). Special chokes for turkey hunting, which requires long range shots at the small head and neck of the bird, can go as high as 1.5 mm (0.059 in). The use of too much choke and a small pattern increases the difficulty of hitting the target; the use of too little choke produces large patterns with insufficient pellet density to reliably break targets or kill game.

As far back as 1787 a Frenchman by the name of M. Magne de Marolles gave an account of choke-boring, though he argued against it.^[3]

Some sources state that the first pioneer was a Czech named Dominik Brandejs, who made shotguns with a choke in order to reduce the dispersion of shots, but his design was not popular in the 1820s.^[4]

The invention of choke boring is usually attributed to American gunsmiths. J.W. Long, in his book American Wildfowling, credits Jeremiah Smith of Southfield, Rhode Island, as the gunsmith who first discovered the concept, as far back as 1827.^[5]

The first known patents for choke boring were granted Sylvester H. Roper, an American inventor and gunsmith.^[6] This was followed by a patent claim in London by W.R. Pape, an English gun maker, whose patent application was six weeks too late to the 1866 Roper patent.

While American gunsmiths were the pioneers of the choke boring system, they had not really progressed beyond the elementary stage and their choked shotguns would lead, throw irregular patterns, and not shoot straight.

W. W. Greener's first intimation of the choke formation was derived from instructions given in a customer's letter, in early 1874. The customer's instructions described a choke but did not give any details on the size or shape nor how it was to be obtained. Hence, Greener had to conduct many experiments to determine the perfect shape and size of a choke for a given bore. After that, he developed tools to produce the choke bore profile correctly and smoothly. The system of choke boring that he pioneered was so successful that it was later adopted by other manufacturers and hence, some authorities give him the credit for inventing the concept, since his method became the first repeatable method of choke boring. William Wellington Greener is thus widely credited as being the inventor of the first practical choke, as documented in his classic 1888 publication, The Gun and its Development.^[7]

In December 1874, the first mention of Greener's choke bore appeared in an article by J.H. Walsh, the editor of Field magazine. The article mentioned the extraordinary shot pattern that the Greener shotgun could produce. The next issue came with an advertisement from Greener, stating that the firm would guarantee that their new guns would shoot a closer pattern than any other manufacturer. The advertisement claimed that Greener 12 bores were warranted to shoot an average pattern of 210, when the best 12 bore gun in the London Gun Trial of 1866 could only average 127. Naturally, the advertisement generated considerable controversy, especially from rival manufacturers of cylinder guns, who refused to believe the numbers quoted in the advertisement.

To resolve the controversy, the editors of Field magazine decided to conduct a public trial in 1875. The London Trial of 1875 pitted choke bores and cylinder guns of various manufacturers in four categories—Class 1 (large bores, any boring), Class 2 (choke bores, 12 gauge), Class 3 (guns of English boring or cylinders) and Class 4 (small gauges, any boring). The choke bored guns performed better than the cylinder guns in all these tests, and W.W. Greener choke bore guns won the class 1, class 2 and class 4 categories. Greener Choke bores also won at the London Gun Trials of 1877 and 1879, and the Chicago Field Gun Trial of 1879. The results of these trials were responsible for making the W.W. Greener name famous, and for confirming the practical advantage of a repeatable method of controlling the performance of a choke on a shotgun.

The choice of choke type has an impact on performance in shooting competitions. At the Shoot-Off at the 2018 Shotgun World Shoot, U.S. shooter Scott Greene and Russian Roman Anikin both fired their last shots almost simultaneously, but Greene had chosen a more constricted barrel choke. This gave a tighter shot pattern and a greater risk of missing, but ultimately led to his target falling faster to win the Shoot-Off.

The exit end of a choke is smaller by some dimension than the actual bore of the barrel. This difference in diameter is the amount of constriction. For example, for a 12 gauge, the bore diameter of the barrel is nominally 18.5 mm (0.73 in), although different manufacturers do vary their as-manufactured bore from this diameter slightly. This is commonly called "overbore", when the as-built diameter exceeds the nominal actual diameter of 18.5 mm (0.73 in). The advantages touted for "overbore" are a perceived reduction in recoil, and a lessened shot deformation for improving shot patterns.

Generally the constriction ranges for chokes will be 0.00-1.15 mm (0.00-0.045 inch) while the constriction for relatively common turkey chokes may be as much as 2.50 mm (0.10 inch). Choke is measured experimentally by observing the percentage of pellets in a charge that impact inside a 75 cm circle at 35 m (25 m for "cylinder" and "Skeet").^[8] Although different choke manufacturers have different identification of their chokes, the notches in the chart below are generally accepted.^[9]

A common method of expressing the amount of constriction is by "points". A "point" is equivalent to 0.025 mm (0.001 in) of constriction of the inner diameter of a choke. Hence, 40 points of constriction would correspond to a constriction of 0.040 inch (≈ 1 mm) in the inner diameter of a choke, corresponding to "Extra Full".

The following list should only be used as a guide, and there may be exceptions.

Older shotgun barrels and chokes were designed for use with lead shot only. Due to changing worldwide waterfowl hunting law restrictions, the use of lead shot has been banned in many parts of the world by international agreement. The reason is that waterfowl hunting with lead shot was identified as a major cause of lead poisoning in waterfowl, which often feed off the bottom of lakes and wetlands where lead shot collects.^[14][15] In the United States, UK, Canada, and many western European countries (France as of 2006), all shot used for waterfowl must now be non-toxic, and therefore may not contain any lead. One method commonly used to work around this legislative change, at least for hunters with newer shotguns with chokes designed for steel shot, is to use steel shot, but the use of steel shot may damage chokes^[16] on older firearms that were designed for use with lead shot only such as Damascus-barreled shotguns. Most non-toxic shot shells produce higher chamber pressures than lead shot and can severely damage these older shotguns and as a rule of thumb, use a more open choke than one would for lead shells. An example would be an improved cylinder choke will perform like a modified choke when shooting steel or tungsten shells. For such hunters, wishing to continue to use older shotguns with chokes not rated for use with steel shot, the use of bismuth shotshells in the place of lead shotshells is common. Other alternatives to lead shotshells than bismuth also exist that are legal for hunting waterfowl, and which are safe for use with older chokes. Within recent years, several companies have created "heavier than lead" non-toxic shot out of tungsten, bismuth, or other elements with a density similar to or greater than lead, and with a shot softness comparable to lead. These shells provide more consistent patterns and greater range than steel shot. They are also generally safe to use in older shotguns with barrels and chokes not rated for use with steel shot, such as for bismuth and tungsten-polymer (although not tungsten-iron) shot. The increase in performance of "heavier than lead" non-toxic shot comes at a higher cost. Boxes of such non-toxic shotshells can cost upwards of $40 (2013) a box for twenty five shells, compared with less than $8 per box (2013) for lead pellet shotshells.

In practice, steel shot patterns as much as two chokes tighter for a given amount of constriction. In other words, a choke that patterns "Modified" with lead or bismuth shot would give a "Full" pattern with steel shot. To avoid excessive wear or grooving from occurring within chokes when shooting steel shot, many manufacturers recommend avoiding shooting steel shot in any chokes marked tighter than "Modified", unless the choke tube is specifically marked as being safe for use with steel shot.^[17]

• Slug barrel