Choke ( firearms ) In firearms , a choke is a tapered constriction of a shotgun barrel ' s bore at the muzzle end . Chokes are almost always used with modern hunting and target shotguns , to improve performance . Their purpose is to shape the spread of the shot in order to gain better range and accuracy . Chokes are variously implemented as either screw - in replaceable chokes , selectable for particular applications , or as fixed , non - replaceable chokes , integral to the shotgun barrel . William Wellington Greener is widely credited as being the inventor of the first practical choke , as documented in his classic 1888 publication , " The Gun and its Development . " 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 , usually 0 . 6 to 0 . 75 inches ( 15 to 19 mm ) . The use of interchangeable chokes allows tuning the performance of a given combination of shotgun and shotshell to achieve a desired level of performance . Function . A choke is designed to alter 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 . Shotguns intended for defensive use often have cylinder or improved cylinder chokes for the widest shot pattern at typically short defensive ranges . The choke should be tailored to the range and size of the targets . A skeet shooter shooting at close targets might use 0 . 005 inches ( 127 micrometres ) of constriction to produce a 30 inch ( 76 cm ) diameter pattern at a distance of 21 yards ( 19 m ) . A trap shooter shooting at distant targets might use 0 . 030 inches ( 762 micrometres ) of constriction to produce a 30 inch ( 76 cm ) diameter pattern at 40 yards ( 37 m ) . Special chokes for turkey hunting , which requires long range shots at the small head and neck of the bird , can go as high as 0 . 060 inches ( 1500 micrometres ) . 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 . " Cylinder barrels " have no constriction . See also : Slug barrel History . The invention of choke boring is usually attributed to American gunsmiths . The first known patents for choke boring were granted Sylvester H . Roper , an American inventor and gunsmith . 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 . J . W . Long , in his book " American Wildfowling " , credits a Jeremiah Smith of Southfield , Rhode Island , as the gunsmith who first discovered the concept , as far back as 1827 . 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 , or 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 . 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 . In order 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 . Constriction . 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 0 . 729 inches , 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 0 . 729 inches . 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 - 0 . 045 inch ( 0 . 00 - 1 . 15 mm ) while the constriction for relatively common turkey chokes may be as much as 0 . 10 inch ( 2 . 50 mm ) . Choke is measured experimentally by observing the percentage of pellets in a charge that impact inside a 30 inch circle at 40 yards ( 25 yards for " cylinder " and " Skeet1 " ) . Although different choke manufacturers have different identification of their chokes , the notches in the chart below are generally accepted . A common method of expressing the amount of constriction is by " points " . A " point " is equivalent to 0 . 001 inch of constriction of the inner diameter of a choke . Hence , 40 points of constriction would correspond to a constriction of 0 . 040 inch in the inner diameter of a choke , corresponding to " Extra Full " . Markings . The marking is usually stamped on the underside of the barrel for older guns without choke tubes , or is spelled out in abbreviated text on the barrel near the gauge marking . In the case of choke tubes , the amount of choke for each barrel is usually stamped on the side of the choke tube , or there may be thin slots cut in the exposed rim of the tube at the barrel opening with the number of slots corresponding to the number of * ' s in this table . ( The 4 wider slots present in the exposed rum of the tube are intended for use with a choke wrench , in the event a choke tube ever becomes stuck in the barrel , and have no relationship to the amount of choke . ) For the case of older , side - by - side , Spanish guns , the choke is often marked on the tang of the barrels , becoming exposed when the wooden forearm is removed , after the double barrels are pivoted off of the break - action , such as when performing a field - stripping and cleaning . The manufacturer ' s mark , along with the proof marks , and the weight of the barrels in grams , and the proof test pressures are also stamped alongside the choke marking on the tang of the barrels , near where the shells are inserted into the barrels . In practice , choke tubes tighter than " Full " , such as " Turkey " , are also commonly marked with but a single thin notch on the end of the tube . Hence , to distinguish between " Full " vs . even tighter chokes , such as " Turkey " , it becomes necessary to measure the bore exit diameter of the choke tube to determine precisely which choke type is present among the " Full " and tighter ( smaller diameter ) chokes . Alternative chokes . Other specialized choke tubes exist as well . Some turkey hunting tubes have constrictions greater than " Turkey " , or additional features like porting to reduce recoil , or " straight rifling " that is designed to stop any spin that the shot column might acquire when traveling down the barrel . These tubes are often extended tubes , meaning they project beyond the end of the bore , giving more room for things like a longer conical section . Shot spreaders or diffusion chokes work opposite of normal chokes — they are designed to spread the shot more than a cylinder bore , generating wider patterns for very short range use . A number of recent spreader chokes , such as the Briley " Diffusion " line , actually use rifling in the choke to spin the shot slightly , creating a wider spread . The Briley Diffusion uses a 1 in 36 cm twist , as does the FABARM Lion Paradox shotgun . Oval chokes , which are designed to provide a shot pattern wider than it is tall , are sometimes found on combat shotgun s , primarily those of the Vietnam War era . They were available for aftermarket addition in the 1970s from companies like A & W Engineering . Military versions of the Ithaca 37 with " duckbill " choke were used in limited numbers during the Vietnam War by US Navy Seals . It arguably increased effectiveness in close range engagements against multiple targets . Two major disadvantages plagued the system . One was erratic patterning . The second was that the shot would spread too quickly providing a very limited effective zone . Offset chokes , where the pattern is intentionally slightly off of center , are used to change the point of impact . For instance , an offset choke can be used to make a double barrelled shotgun with poorly aligned barrels hit the same spot with both barrels . For shotguns with fixed chokes integral to the barrel , it is sometimes still possible to change the choke to a tighter choke . This is done by increasing the diameter of the bore inside the barrel for a short length of barrel , while the portion of the barrel and bore nearest the muzzle is left as it was . The effect is to form what is called a " jug choke " or a " reverse choke " . This method is sometimes used by gunsmiths to implement a tighter choke on an existing gun without replaceable chokes , and can be done without requiring replacement of a barrel and without installing new screw - in replaceable chokes . Advantages claimed for a " jug choke " include improved patterns , reduction of recoil , and an increase of choke in a shotgun that does not have replaceable choke tubes . When a " jug choke " is implemented in an existing choked barrel , the bore inside the bored - out section of barrel effectively becomes " overbored " , and this also typically lessens the amount of shot deformation , thereby increasing the shot pattern density . Steel shot and alternatives . 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 . 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 on older firearms that were designed for use with lead shot , only . 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 than steel shot and provide 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 .