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Shotgun shell
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A shotgun shell (shotshell) is a self-contained cartridge loaded with shot or a slug designed to be fired from a shotgun. Most shotgun shells are designed to be fired from a smoothbore barrel, but with the recent gain in popularity of dedicated shotguns with rifled barrels for firing slugs, there are many rounds specifically designed to be fired from a rifled barrel.
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A shotgun shell (shotshell) is a self-contained cartridge loaded with shot or a slug designed to be fired from a shotgun. Most shotgun shells are designed to be fired from a smoothbore barrel, but with the recent gain in popularity of dedicated shotguns with rifled barrels for firing slugs, there are many rounds specifically designed to be fired from a rifled barrel. A rifled barrel will increase the accuracy of the shotgun with slugs, but makes it unsuitable for firing shot, as the rifling causes the shot to form a hollow "O" shape in flight. Some less lethal shotgun ammunition is available in the form of slugs made of low-density material, such as rubber. See shotgun specialty ammunition for more information.
Construction of a typical shotshell
Modern shotgun cartridges typically consist of a plastic case, with the base covered in a thin brass covering. Paper shells used to be common, and are still made, as are solid brass shells. Some companies have produced what appear to be all-plastic shells, although in these there is a small metal ring cast into the rim of the shell to provide strength. Often the more powerful loads will use "high brass" shells, with the brass extended up further along the sides of the shell, while light loads will use "low brass" shells. The brass does not actually provide a significant amount of strength, but the difference in appearance provides shooters with a way to quickly differentiate between high and low powered ammunition.
The base of the shotshell is fairly thick to hold the large shotgun primer, which is quite a bit longer than primers used for rifle and pistol ammunition. Modern smokeless powders are far more efficient than the original black powder used in shotgun shells, so very little space is actually taken by powder; shotguns use small quantities of powerful double base powders, equivalent to fast-burning pistol powders, with up to 50% nitroglycerin. After the powder comes the wadding. The primary purpose of a wad is to provide a gas seal, since without a wad the gas would just blow through the shot rather than propelling it. The wad consists of three parts, the powder wad, the cushion, and the shot cup, which may be separate pieces or be one part. The powder wad acts as the gas seal, and is placed firmly over the powder; it may be a paper or plastic part. The cushion comes next, and it is designed to compress under pressure, to act as a shock absorber and minimize the deformation of the shot; it also serves to take up as much space as is needed between the powder wad and the shot. Cushions are almost universally made of plastic with crumple zones. The shot cup is the last part of the shell, and it serves to hold the shot together as it moves down the barrel. Shot cups have slits on the sides so that they peel open after leaving the barrel, allowing the shot to continue on in flight undisturbed. Shot cups are also almost universally plastic. The shot fills the shot cup (which must be of the correct length to hold the desired quantity of shot), and the shotgun shell is then crimped closed.
Shotshell sizes
Shotgun shells are generally measured by "gauge," though in Britain and some other locations outside the United States, the term "bore" is used with the same meaning. While higher numbers incrementally increase the width of a rifle barrel, gauge refers to the division of a pound of lead into equal sized spheres: the fewer number of spheres the pound is divided into, the greater the diameter each will be.
For example, a shotgun is called 12 gauge because twelve lead spheres, each of which just fits the inside diameter of the barrel, weigh one pound. The gauge is a count of the number of lead spheres with a total weight of one pound (454 g), each sphere fitting smoothly into the barrel. This measurement comes from the time when early cannons were designated in a similar manner—a "12 pounder" would be a cannon that fired a 12 pound (5.5 kg) cannonball; inversely, an individual "12 gauge" shot would in fact be a 1/12 pounder (38 g).
The most popular shotgun gauge by far is 12 gauge, with other common gauges being 10, 16, 20, and 28. There are also some shotguns measured by diameter, rather than gauge, these are the .410 (10 mm), 9 mm (.357), and .22 (5.5 mm); these are correctly called ".410 bore", not ".410 gauge".
The .410 bore is the smallest shotgun size you will typically find commercially available. For size comparison purposes the .410 bore, when measured by gauge, would be around 67 or 68 gauge (mathematically it's 67.62 gauge), not the sometimes mistakenly assigned 36 gauge.
Shotshells are also found in some handgun cartridges, such as .38 Special and .44 Magnum; these are often used in revolvers for defense against snakes at very close ranges, or for killing small pests such as rats. A number of single shot pistols and rifles are made in .45 Colt with special screw-in chokes allowing the use of .410 bore shells—usually the chokes are designed with deep grooves parallel to the bore designed to stop the spin of the shot column, as the .45 Colt barrel is rifled. Taurus recently introduced a revolver named The Judge, which accommodates both .45 Colt and .410 shotshells.
Shotgun Gauge Diameter Formulas
The first formula “Standard Shotgun Gauge Diameter Formula” assumes the standard gravity for lead is 11.3523g/cm3 (see "Formula Research Notation" below to explain), and the second formula “Shotgun Gauge Diameter Formula 2” allows you to choose your own specific gravity for lead.
Standard Shotgun Gauge Diameter Formula
The following formula will accurately calculate the diameter (dn) in inches of any shotgun gauge size (n).
or
This table was created using this formula and 25.4mm per inch conversion:
| Gauge | Diameter Inch | Diamter mm |
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| A | 2.0000" | 50.800mm | | 1.5 | 1.4588" | 37.054mm | | 2 | 1.3254" | 33.665mm | | 3 | 1.1579" | 29.409mm | | 4 | 1.0520" | 26.720mm | | 8 | 0.8350" | 21.208mm | | 10 | 0.7751" | 19.688mm | | 12 | 0.7294" | 18.527mm | | 13 | 0.7102" | 18.039mm | | 14 | 0.6929" | 17.599mm | | 16 | 0.6627" | 16.833mm | | 20 | 0.6152" | 15.626mm | | 24 | 0.5789" | 14.705mm | | 28 | 0.5499" | 13.968mm | | 32 | 0.5260" | 13.360mm | | 67.56636 | 0.4100" | 10.414mm |
Formula Research Notation
Because the shotgun gauge size (n) equals the sphere size of lead (a lead ball) that weights 1/n lbs (a dozen 12 gauge lead balls weigh 1 lb. all together), specific gravity (density) of lead needs to be incorporated into the formula. However specific gravity for lead varies from 11.34 to 11.39 based on various lists. Lead contains impurities when it is mined and this causes its specific gravity to typically fall below 11.34 until it is purified. Lead ingots and cast lead will have the lowest specific gravity, while rolled lead will have higher density (highest specific gravity).
The formula uses a specific gravity (density) of 11.3523g/cm3 because it showed the greatest decimal accuracy in research, whereas most other official records document 11.352 (and many round to 11.35) as the specific gravity for ingots and cast lead.
Assuming the specific gravity is accurate, mathematically, the formula is precise up to 1/1000th of an inch (three decimal places), and the fourth decimal place (1/10,000th of an inch) is fairly accurate because of the decimal precision used to develop it; e.g. Specific gravity (density) for lead metal is 11.3523g/cm3 and 1/0.0254" per meter (39.3700787401575” per meter) was used to determine 16.387064cm3 per inch3, resulting in lead metal weighing 0.41012785697255 lbs/in3.
Shotgun Gauge Diameter Formula 2
For any shotgun gauge size (n), the following formula is used if you want to specify an alternate specific gravity (s) for lead in the calculation; e.g. 11.34 or 11.35 are the most common because they round to the nearest two decimal places.
or
Another source for a gauge size formula can be found at Gauge (bore diameter).
Shot sizes
Shotshells are loaded with different sizes of shot depending on the target. For skeet shooting, a small shot such as a # 8 or #9 would be used, because range is short and a high density pattern is desirable. Trap shooting requires longer shots, and so a larger shot, up to #7½ would be desired. For hunting game, the range and the penetration needed to assure a clean kill must both be considered. Shot loses its velocity very quickly due to its low sectional density (see external ballistics). Small shot, like that used for skeet and trap, will have lost all appreciable energy by 100 yards or meters, which is why trap and skeet ranges can be located in relatively close proximity to inhabited areas with negligible risk of injury to those outside the range.
Birdshot
Birdshot sizes are numbered similar to the shotgun gauges; the smaller the number, the larger the shot. Generally birdshot is just called "shot", such as "number 9 shot" or "BB shot". A useful method for remembering the diameter of numbered birdshot is simply to subtract the shot size from 17. The resulting answer is the diameter of the shot in hundredths of an inch. For example, number 2 shot gives 17-2 = 15, meaning that the diameter of number 2 shot is 15/100 or 0.15". B shot is .170 inches, and sizes go up in .01 increments for BB and BBB.
Birdshot selection
For hunting, shot size must be chosen not only for the range, but also for the game. The shot must reach the target with enough energy to penetrate to a depth sufficient to kill the game. Lead shot is still the best performer for the money, but environmental restrictions on the use of lead, especially with waterfowl, require steel, bismuth, or tungsten composites. Steel, being significantly less dense than lead, requires larger shot sizes, but is a good choice when cost is a consideration. Steel, however, cannot safely be used in some older shotguns without causing damage to either the bore or to the choke of the shotgun due to the hardness of steel shot. Tungsten shot is equal or even greater in density than lead, but is far more expensive. Bismuth shot falls in between steel and tungsten shot in both density and cost.
Buckshot
Larger sizes of shot, large enough that they must be carefully packed into the shell rather than simply dumped or poured in, are called "buckshot." Buckshot is used for hunting larger game, such as deer (hence derivation of the name), and also in riot shotguns and combat shotguns for defensive, police, and military use. Buckshot is also categorized by number, with smaller numbers being larger shot. It is called either "buckshot" or just "buck", such as "triple-ought buck" or "number 4 buck".
Shotshells and patterning
Most modern sporting shotguns have interchangeable choke tubes to allow the shooter to change the spread of shot out of the gun. In some cases, it is not practical to do this; the gun might have fixed choke, or a shooter firing at receding targets may want to fire a wide pattern immediately followed by a narrower pattern out of a single barrelled shotgun. The spread of the shot can also be altered by changing the characteristics of the shell.
Narrower patterns
A buffering material, such as granulated plastic, sawdust, or similar material can be mixed with the shot to fill the spaces between the individual pellets. When fired, the buffering material compresses and supports the shot, reducing the deformation the shot pellets experience under the extreme acceleration. Copper plated lead shot, steel, bismuth, and tungsten composite shot all have a hardness greater than that of plain lead shot, and will deform less as well. Reducing the deformation will result in tighter patterns, as the spherical pellets tend to fly straighter.
Wider patterns
Shooting the softest possible shot will result in more shot deformation and a wider pattern. This is often the case with cheap ammunition, as the lead used will have minimal alloying elements and be very soft. Spreader wads are wads that have a small plastic or paper insert in the middle of the shot cup, usually a cylinder or "X" cross-section. When the shot exits the barrel, the insert helps to push the shot out from the center, opening up the pattern. Often these result in inconsistent performance, though modern designs are doing much better than the traditional improvised solutions. Intentionally deformed shot (hammered into ellipsoidal shape) or cubical shot will also result in a wider pattern, much wider than spherical shot, with more consistency than spreader wads. Spreader wads and non-spherical shot are disallowed in some competitions. Hunting loads that use either spreaders or non-spherical shot are usually called "brush loads", and are favored for hunting in areas where dense cover keeps shot distances very short.
See also
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