Shotgun slug

The mass of a shotgun slug is kept within SAAMI pressure limits for shot loads in any given shotgun shell load design. Slugs are designed to pass safely through open chokes and should never be fired through tight (or unknown) barrels. The internal pressure of the shotshell load will actually be slightly higher than the equivalent mass slug projectile load, due to an increased resistance that occurs from a phenomenon known as shot setback. Common 12 gauge slug masses are 7⁄8 oz, 1 oz, and 1+1⁄8 oz, the same as common birdshot payloads.[4][5]

A 1 oz. (437.5 grain) 2 3/4-inch Foster 12 gauge shotgun slug achieves a velocity of approximately 1,560 fps (475 m/s) with a muzzle energy of 2,363 ft. lbs (3204 J). 3-inch slugs travel at around 1,760 fps with a muzzle energy of 3,105 ft-lbs.[6] In contrast, a .30-06 bullet weighing 150 grains at a velocity of 2,600 fps achieves an energy of 2,250 ft-lbs. A 180 grains bullet at 2,775 feet per second, which is a very common 30-06 load and not its true maximum potential, achieves 3,079 ft-lbs of energy. Due to the slug's larger caliber and shape, it has greater air resistance and slows much more quickly than a bullet. It slows to less than half its muzzle energy at 100 yards, which is below the minimum recommended energy threshold for large game (1,000 ft-lbs is recommended for deer, 1,500 ft-lbs for elk, and 2,000 ft-lbs for moose). A slug also becomes increasingly inaccurate with distance; out to 100 yards, it drops approximately five inches and has a maximum range of approximately 400 yards. In contrast, centerfire projectiles from rifles can travel anywhere from 300-1000+ yards. Shotgun slugs are best suited for uses over short ranges.

The Taylor knock-out factor (TKOF) was developed as a measure of stopping power for hunting game, however it is a rather flawed calculation.[7] It is defined as the product of bullet mass, velocity and diameter, using the imperial units grains (equal to 64.79891 mg), feet per second (equal to 0.3048 m/s) and inches (equal to 25.4 mm):

${\displaystyle {\text{TKOF}}={\text{Mass(grains)}}\times {\text{Velocity(feet per second)}}\times {\text{Diameter(inches)}}\times {\frac {1}{7000}}.}$

Some TKOF example values for shotgun slugs are:

• 71 TKOF for a 70 mm (2 3/4 in) slug (i.e. 437.5 grain (1 oz) × 1,560 FPS × 0.729 caliber /7000 = 71.07 TKOF)
• 80 TKOF for a 76 mm (3.00 in) slug (i.e. 437.5 grain (1 oz) × 1,760 FPS × 0.729 caliber /7000 = 80.19 TKOF)

To compare with rifles, some TKOF example values for rifle cartridges are:

Full-bore slugs, such as the Brenneke and Foster types, contrary to popular belief, use a spin-stabilization method of stabilization through the use of angled fins on the slug’s outer walls. The slight 750 RPM spin is enough to stabilize the slug because the slug’s center of pressure is so much further back than its center of mass.[8] Saboted slugs are similar in shape to handgun bullets and airguns slugs. Their center of pressure is in front of their center of mass, meaning a higher twist rate is required to achieve proper stabilization. Most saboted slugs are designed for rifled shotgun barrels and are stabilized through gyroscopic forces from their spin.[9][10]

A 12 gauge Brenneke slug

The Brenneke slug was developed by the German gun and ammunition designer Wilhelm Brenneke (1865–1951) in 1898. The original Brenneke slug is a solid lead slug with ribs cast onto the outside, much like a rifled Foster slug.[11] There is a plastic, felt or cellulose fiber wad attached to the base that remains attached after firing. This wad serves as a gas seal, preventing the gasses from going around the projectile. The lead "ribs" that are used for inducing spin also swage through any choked bore from improved cylinder to full. The soft metal, typically lead, fins squish or swage through the choke to allow for an easy passage.[12]

A Remington 870 12 gauge with sighted cylinder bore barrel suitable for Foster slugs and buckshot

The "Foster slug", invented by Karl M. Foster in 1931, and patented in 1947 (U.S. Patent 2,414,863), is a type of shotgun slug designed to be fired through a smoothbore shotgun barrel, even though it commonly labeled as a "rifled" slug. A rifled slug is for smooth bores and a sabot slug is for rifled barrels.[9]

Most Foster slugs also have "rifling", which consists of ribs on the outside of the slug. Like the Brenneke, these ribs impart a rotation on the slug to correct for manufacturing irregularities, thus improving precision (i.e., group size.)[13] Similar traditional rifling, the rotation of the slug imparts gyroscopic stabilization. The "Denver Pile Driver" slug is a foster slug without rifled fins. When tested the slug tumbled and was not stable or accurate past 25 yards, proving foster slugs do need some form of spin stabilization. [14]

Saboted slugs are shotgun projectiles smaller than the bore of the shotgun and supported by a plastic sabot. The sabot is traditionally designed to engage the rifling in a rifled shotgun barrel and impart a ballistic spin onto the projectile. This differentiates them from traditional slugs, which are not designed to benefit from a rifled barrel (though neither does the other any damage). Due to the fact that they do not contact the bore, they can be made from a variety of materials including lead, copper, brass, or steel. Saboted slugs can vary in shape, but are typically bullet-shaped for increased ballistic coefficient and greater range. The sabot is generally plastic and serves to seal the bore and keep the slug centered in the barrel while it rotates with the rifling. The sabot separates from the slug after it departs the muzzle. Saboted slugs fired from rifled bores are superior in accuracy over any smooth-bored slug options with accuracy approaching that of low-velocity rifle calibers.[9]

From the left, plumbata discarding sabot (No. 1); plumbata slugs (No. 2, No. 5); wad slug (No. 3), sabot slugs (No. 3, No. 4)

A modern variant between the Foster slug and the sabot slug is the wad slug. This is a type of shotgun slug designed to be fired through a smoothbore shotgun barrel. Like the traditional Foster slug, a deep hollow is located in the rear of this slug, which serves to retain the center of mass near the front tip of the slug much like the Foster slug. However, unlike the Foster slug, a wad slug additionally has a key or web wall molded across the deep hollow, spanning the hollow, which serves to increase the structural integrity of the slug while also reducing the amount of expansion of the slug when fired, reducing the stress on the shot wad in which it rides down a barrel. Also, unlike Foster slugs that have thin fins on the outside of the slug, much like those on the Brenneke, the wad slug is shaped with an ogive or bullet shape, with a smooth outer surface. The wad slug is loaded using a standard shotshell wad, which acts like a sabot. The diameter of the wad slug is slightly less than the nominal bore diameter, being around 0.690 inch (17.5 mm) for a 12-gauge wad slug, and a wad slug is generally cast solely from pure lead, necessary for increasing safety if the slug is ever fired through a choked shotgun. Common 12 gauge slug masses are 7⁄8 oz, 1 oz, and 1+1⁄8 oz, the same as common birdshot payloads. Depending on the specific stack-up, a card wad is also sometimes located between the slug and the shotshell wad, depending largely on which hull is specified, with the primary intended purpose of improving fold crimps on the loaded wad slug shell that serves to regulate fired shotshell pressures and improve accuracy.

It is also possible to fire a wad slug through rifled slug barrels, and, unlike with the Foster slug where lead fouling is often a problem, a wad slug typically causes no significant leading, being nested inside a traditional shotshell wad functioning as a sabot as it travels down the shotgun barrel.

Accuracy of wad slugs falls off quickly at ranges beyond 75 yards (70 m), thereby largely equaling the ranges possible with Foster slugs, while still not reaching the ranges possible with traditional sabot slugs using thicker-walled sabots.

Unlike the Foster slug which is traditionally roll-crimped, the wad slug is fold-crimped. Because of this important difference, and because it uses standard shotshell wads, a wad slug can easily be reloaded using any standard modern shotshell reloading press without requiring specialized roll-crimp tools.

A plumbata slug has a plastic stabilizer attached to the projectile. The stabilizer may be fitted into a cavity in the bottom of the slug, or it may fit over the slug and into external notches on the slug. With the first method discarding sabots may be added. And with the second, the stabilizer may act as a sabot, but remains attached to the projectile and is commonly known as an "Impact Discarding Sabot" (IDS).[15]

There are some types of all-steel subcaliber slugs supported by a protective plastic sabot (the projectile would damage the barrel without a sabot). Examples include Russian "Tandem" wadcutter-type slug (the name is historical, as early versions consisted of two spherical steel balls) and ogive "UDAR" ("Strike") slug and French spool-like "Balle Blondeau" (Blondeau slug) and "Balle fleche Sauvestre" (Sauvestre flechette) with steel sabot inside expanding copper body and plastic rear empennage. Made of non-deforming steel, these slugs are well-suited to shooting in brush, but may produce overpenetration. They also may be used for disabling vehicles by firing in the engine compartment or for defeating hard body armor.

Another variant of a Great Depression–era shotgun slug design is the wax slug. These were made by hand by cutting the end off a standard birdshot loaded shotshell, shortening the shell very slightly, pouring the lead shot out, and melting paraffin, candle wax, or crayons in a pan on a stovetop, mixing the lead birdshot in the melted wax, and then using a spoon to pour the liquified wax containing part of the birdshot back into the shotshell, all while not overfilling the shotgun shell. Once the shell cooled, the birdshot was now held in a mass by the cooled paraffin, and formed a slug. No roll or fold crimp was required to hold the wax slug in the hull. These were often used to hunt deer during the Depression.

Yet another expedient shotgun slug design is the cut shell. These are made by hand from a standard birdshot shell by cutting a ring around and through the hull of the shell that nearly encircles the shell, with the cut traditionally located in the middle of the wad separating the powder and shot. A small amount of the shell wall is retained, amounting to roughly a quarter of the circumference of the shotshell hull. When fired, the end of the hull separates from the base and travels through the bore and down range. Cut shells have the advantage of expedience. They can be handmade on the spot as the need arises while on a hunt for small game if a larger game animal such as a deer or a bear appears. In terms of safety, part of the shell may remain behind in the barrel, causing potential problems if not noticed and cleared before another shot is fired.[16][17]

Large game (including deer and wild boar) hunting is only allowed with large caliber rifles; shotguns are only allowed for small and medium-sized game, up to foxes and geese. However, when a shotgun has a rifled barrel, it is considered a rifle, and it becomes legal for hunting roe deer, minimum caliber 5.56 mm and 980 joules at a 100 meters, and deer and wild boar, minimum caliber 6.5 mm and 2200 joules at 100 meters.

Slugs fired from a single-barrel shotgun are allowed for hunting wild boar, fallow deer, and mouflon, although when hunting for wounded game there are no restrictions. The shot must be fired at a range of no more than 40 meters. The hunter must also have the legal right to use a rifle for such game in order to hunt with shotgun slugs.

Ammunition which contains no fewer than five projectiles, none of which exceed 0.36 inch (9 mm) in diameter, is legal with a Section 2 Shotgun Certificate. Slugs, which contain only one projectile and usually exceed 0.36 inch in diameter, are controlled under the Firearms Act, and require a firearms certificate to possess, which is very strictly regulated. Legal uses in the UK include, but are not restricted to, practical shotgun enthusiasts as members of clubs and at competitions, such as those run by or affiliated to the UKPSA.[21]

Rifled barrels for shotguns are an unusual legal issue in the United States of America. Firearms with rifled barrels are designed to fire single projectiles, and a firearm that is designed to fire a single projectile with a diameter greater than .50 caliber (12.7 mm) is considered a destructive device and as such is severely restricted. However, the ATF has ruled that as long as the gun was designed to fire shot, and modified (by the user or the manufacturer) to fire single projectiles with the addition of a rifled barrel, then the firearm is still considered a shotgun and not a destructive device.

In some areas, rifles are prohibited for hunting animals such as deer. This is generally due to safety concerns. Shotgun slugs have a far shorter maximum range than most rifle cartridges, and are safer for use near populated areas. In other areas, there are special shotgun-only seasons for deer. This may include a modern slug shotgun, with rifled barrel and high performance sabot slugs, which provides rifle-like power and accuracy at ranges over 150 yards (140 m).