Editing Shotgun (section)

===Pattern and choke===<!-- This section is linked from [[Flintlock]] -->
{{main|Choke (firearms)}}
{{See also|Slug barrel}}
Shot, small and round and delivered without spin, is ballistically inefficient. As the shot leaves the barrel it begins to disperse in the air. The resulting cloud of pellets is known as the shot [[pattern]], or [[shotgun shot spread]]. The ideal pattern would be a circle with an even distribution of shot throughout, with a density sufficient to ensure enough pellets will intersect the target to achieve the desired result, such as a kill when hunting or a break when shooting clay targets. In reality the pattern is closer to a [[Normal distribution|Gaussian]], or normal distribution, with a higher density in the center that tapers off at the edges. Patterns are usually measured by firing at a {{convert|30|in|cm|adj=on}} diameter circle on a large sheet of paper placed at varying distances. The hits inside the circle are counted, and compared to the total number of pellets, and the density of the pattern inside the circle is examined. An "ideal" pattern would put nearly 100% of the pellets in the circle and would have no voids—any region where a target silhouette will fit and not cover 3 or more holes is considered a potential problem.

A constriction in the end of the barrel known as the [[Choke (firearms)|choke]] is used to tailor the pattern for different purposes. Chokes may either be formed as part of the barrel at the time of manufacture, by squeezing the end of the bore down over a [[mandrel]], or by threading the barrel and screwing in an interchangeable choke tube. The 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, a maker of interchangeable shotgun chokes, uses a conical portion about three times the bore diameter in length, so the shot is gradually squeezed down with minimal deformation. The cylindrical section is shorter, usually {{convert|0.6|to|0.75|in|mm|abbr=off}}. The use of interchangeable chokes has made it easy to tune the performance of a given combination of shotgun and shotshell to achieve the desired performance.

The choke should be tailored to the range and size of the targets. A skeet shooter shooting at close targets might use 127 micrometres (0.005&nbsp;inches) of constriction to produce a {{convert|76|cm|in|abbr=on}} diameter pattern at a distance of {{convert|19|m|yd|abbr=on}}. A [[trap shooting|trap]] shooter shooting at distant targets might use 762 micrometres (0.030&nbsp;inches) of constriction to produce a {{convert|76|cm|in|abbr=on}} diameter pattern at {{convert|37|m|yd|abbr=on}}. Special chokes for turkey hunting, which requires long range shots at the small head and neck of the bird, can go as high as 1500 micrometres (0.060&nbsp;inches). The use of too much choke and a small pattern increases the difficulty of hitting the target, whereas 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.

{| class="wikitable" style="text-align:center;"
|+ Table of shotgun chokes<br />for a 12-gauge shotgun using lead shot
|-
!Constriction<br />(micrometres)
!Constriction<br />(inches)
!American name
!British name
!Percentage of shot<br />in a {{convert|76|cm|in|abbr=on}} circle <br />at {{convert|37|m|yd|abbr=on}}
!Total spread at 37 m <br /> (cm)
!Total spread at 40 yds <br /> (in)
!Effective range<br /> (m)
!Effective range<br /> (yd)
|-
| 0|| .000|| Cylinder|| || 40|| 150|| 59|| 18||20
|-
| 127|| .005|| Skeet|| {{frac|1|8}}|| 45|| 132|| 52|| 21||23
|-
| 254|| .010|| Improved Cylinder|| {{frac|1|4}}|| 50|| 124|| 49|| 23||25
|-
| 381|| .015|| Light Modified|| || &nbsp;|| &nbsp;|| &nbsp;|| &nbsp;|| &nbsp;
|-
| 508|| .020|| Modified|| {{frac|1|2}}|| 60|| 117|| 46|| 32||35
|-
| 635|| .025|| Improved Modified|| {{frac|3|4}}|| &nbsp;|| &nbsp;|| &nbsp;|| &nbsp;|| &nbsp;
|-
| 762|| .030|| Light Full|| || &nbsp;|| 109|| 43|| &nbsp; || &nbsp;
|-
| 889|| .035|| Full|| Full|| 70|| &nbsp;|| &nbsp;|| 37||40
|-
| 1143|| .045|| Extra Full|| || &nbsp;|| &nbsp;|| &nbsp;|| &nbsp;|| &nbsp;
|-
| 1270|| .050|| Super Full|| || &nbsp;|| &nbsp;|| &nbsp;|| &nbsp;|| &nbsp;
|}

Other specialized choke tubes exist as well. Some turkey hunting tubes have constrictions greater than "Super Full", 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&nbsp;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.<ref>{{cite book|title=The Police Shotgun Manual|publisher=Thomas|author=Roger H. Robinson|year=1973|isbn=978-0-398-02630-1|pages=91–94}}</ref> 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 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.