Shotgun slug

A modern shotgun slug is a heavy lead or copper-covered lead projectile, with or without a plastic tip, that may have pre-cut rifling and that is intended for use in a shotgun and often used for hunting large game. The first effective modern shotgun slug was introduced by Wilhelm Brenneke in 1898, and his design remains in use today. Most shotgun slugs are designed to be fired through a choked smoothbore barrel; they must be self-stabilizing in the absence of rifling.

Earlier types of slugs were also used in 4 bore guns intended for hunting dangerous African game in the 19th Century. Such 4 bore guns were produced in both rifled and non-rifled versions. Similarly, modern shotguns have been produced with rifled barrels, or rifled choke tubes, and slugs designed to be fired from them use spin stabilization. As these specialized shotguns are far more accurate than a smoothbore gun, they also usually have a mount for an optical sight, such as a scope. Many of these slugs are saboted sub-caliber projectiles, resulting in greatly improved external ballistics performance.

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.

A shotgun slug is typically far more massive than a rifle bullet. One common .30-06 bullet, for example, weighs 150 grains. The lightest common 12 gauge shotgun slug is the 7/8-oz slug, which weighs approximately 383 grains.

Uses
Shotgun slugs are used to provide rifle-like performance, but not range, from a shotgun, by firing a single large projectile rather than a large number of smaller ones. In many populated areas, hunters are restricted to shotguns even for medium to large game, such as deer, due to concerns about the range of modern rifle bullets. In such cases a slug will provide a longer range than a load of buckshot, which traditionally was used at ranges up to approximately 25 yards (22.8m), without approaching the range of a rifle.

Less lethal slugs are often used as the main ammunition for police riot shotguns. The slugs will provide accuracy sufficient for antipersonnel use out to ranges of about 100 yd. This allows the officer the ability to use the shotgun as a reasonable substitute for a rifle at medium ranges. The shotgun allows the operator to quickly eject a chambered buckshot cartridge and replace it with a less-lethal cartridge such as a bean bag round, instead of being required to unload the magazine as with most rifles or handguns, allowing the officer to adapt to changing situations in switching between lethal or a less lethal ammunition for the same firearm.

Slug masses
The mass of a shotgun slug projectile is always kept under the mass limit of the maximum recommended SAAMI pressure limited shot loads in any given shotgun shell load design. Provided that the wad column is adjusted to enable a proper crimp with the recommended powder column weight, and that the stiffness of the wad column is equivalent to the parent shotshell load, and that the maximum diameter of the slug and sabot is kept within maximum SAAMI shotgun slug diameter limits to enable the slug/sabot combination to pass safely through any choke, the maximum pressure of the slug shell will not exceed the pressure of an equivalent payload mass shotshell load. 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. Because of this equivalence, common slug masses are 7/8 oz, 1-oz, and 1-1/8 oz, the same as common shotgun shell shot payloads. Slightly heavier slugs have been manufactured and sold, and are safe, being held within SAAMI limits, but are rarely used as they generate more recoil than most shooters will accept.

Comparisons with rifle bullets
Shotgun slugs (12 gauge) achieve typical velocities of approximately 1800 fps for 1-oz. (437.5 grain) slugs, for an energy of over 3,100 ft-lbs. In contrast, a .30-06 bullet weighing 150 grains at a velocity of 2600 fps achieves an energy of 2,250 ft-lbs. However, a shotgun slug sheds velocity more quickly than a spitzer bullet fired in a rifle. At 100 yards, a shotgun slug will have slowed significantly, whereas a spitzer bullet will still retain much of the velocity that it had at 25 yards. Shotgun slugs thus are best suited for uses over shorter ranges than rifle bullets. However, for hunting in built-up populated areas, shotgun slugs are considerably safer than rifles, with maximum ranges typically under 400 yards, in comparison to maximum ranges of over 3.5 miles for errant projectiles from rifles.

Types
The earliest shotgun slugs were just lead balls, of just under the bore diameter, allowing them to pass through the barrel. Often called "pumpkin balls", "punkin balls", or "pumpkin nuts", these slugs had very poor accuracy and were only effective at very close range, where they could be relied on to hit the target in a vital area. In essence, they enabled a shotgun to become the equivalent of a musket. Later types of slugs, such as the Brenneke design, use a weight-based design and fins to provide spin stability together with the ability to easily compress and pass through a choked barrel. These can be fired through a smoothbore barrel with reasonable accuracy, and significantly extend the effective range of a shotgun slug. The latest improvement is the saboted slug fired from a rifled shotgun barrel. The saboted slug and rifled barrel combination provides even greater accuracy than the rifled slugs, and the slugs themselves are more aerodynamic, providing more range and a flatter trajectory.

Brenneke slugs
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 fins cast onto the outside, much like a rifled Foster slug. There is a plastic, felt or cellulose fiber wad attached to the base that remains attached after firing. This wad serves both as a gas seal and as a form of drag stabilization, much like the mass-forward design of the Foster slug. The "fins" impart little or no spin to the projectile; their actual purpose is to decrease the bearing surface of the slug to the barrel and therefore reduce friction and increase velocity.

Since the Brenneke slug is solid, rather than hollow like the Foster slug, the Brenneke will generally deform less on impact and provide deeper penetration (see terminal ballistics). The sharp shoulder and flat front of the Brenneke (similar in dimensions to a wadcutter bullet) mean that its external ballistics restrict it to short-range use, as its accuracy is similar to that of an American Foster slug while retaining the improved penetration and slug integrity of the Brenneke design.

Foster slugs
A Foster slug, invented by Karl Foster (or Forster) in 1931, is a type of shotgun slug designed to be fired through a smoothbore shotgun barrel. It was invented by Foster to enable deer hunting in the Great Depression using smoothbore, choked shotguns. Foster cast them by hand from soft lead, filed grooves on their exteriors, and sold them to his neighbors to feed their families. The Foster is the standard American domestic shotgun slug; they are sometimes referred to as "American slugs" to differentiate them from the standard "European slug" design popularized earlier by Brenneke.

The defining characteristic of the Foster slug is the deep hollow in the rear, which places the center of mass very near the front tip of the slug, much like a shuttlecock or a pellet from an airgun. If the slug begins to tumble in flight, drag will tend to push the slug back into straight flight, stabilizing the slug. This gives the Foster slug stability and allows for accurate shooting through smoothbore barrels out to ranges of about 75 yd, providing accuracy over far greater distances than the 25 yd. limit typical when shooting traditional "pumpkin balls" through a shotgun.

Most Foster slugs also have "rifling", which consists of thin fins on the outside of the slug. Contrary to popular belief, these fins impart no spin onto the slug as it travels through the air. The actual purpose of these fins is to minimize the friction on both the barrel and projectile and allow the slug to be swaged down safely when fired through a choke, although accuracy will suffer and choke wear may be progressively accelerated when fired through any gauge choked tighter than about improved cylinder. Foster slugs can safely be swaged down much more than Brenneke slugs, when fired through a choke, being hollow. The amount of wear on a choke is therefore much less of a problem than when shooting Brenneke slugs.

It is also possible to fire Foster slugs through rifled slug barrels, though lead fouling (build-up in the rifle grooves) is a problem. Accuracy is otherwise not appreciably affected in standard shotgun rifling.

Roll-crimping is traditionally used to close a shotgun shell containing a Foster slug. This increases the difficulty for handloading Foster Slugs, today, as special roll-crimp tools using a drill press are often recommended for handloading Foster Slugs. During the 1930s, though, many if not most shotgun shells were roll-crimped over an overshot card, and hand tools for putting a roll crimp on a paper shell were readily available and very inexpensive. Using a roll-crimp was simply the easiest way to close a shotgun shell case at the time.

Wax slugs
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 or candle wax 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. Accuracy was generally good up to distances of approximately 50 yards, midway between the accuracy range limits of a "pumpkin ball" and a Foster slug. These were often used to poach deer during the Depression. Careful cleaning after firing a wax slug was necessary, to avoid wax build-up from occurring, potentially causing a dangerous barrel obstruction.

Cut shell slugs
Yet another variant of a Great Depression era shotgun slug design is the cut shell. These were made by hand from a standard birdshot shotshell 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. Usually, just a small amount of the wall of the original shell was retained, amounting to roughly 1/4 of the circumference of the shotshell hull. When fired in a single-shot or double-barreled break-action shotgun, the end of the shotshell hull separates from the head of the shell, and travels down range. Cut shells generally provided accuracy up to distances of approximately 35 to 40 yards, all while avoiding the need to cast or fire a "pumpkin ball". They were often handmade on the spot as the need arose while on a hunt for small game, if a larger game animal such as a deer or a bear appeared. Cut shell slugs were often used to add to the larder during the Great Depression. These cut shell slugs can be dangerous, as they are thought to cause higher chamber pressures. Part of the shell may remain behind in the barrel, causing problems if not noticed and cleared before another shot is fired.

Wad slugs
A modern variant between the Foster slug and the saboted slug is the wad slug, sometimes called a "drive key slug", or just a "key 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 moulded 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-inches 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 wad slug masses include 7/8-oz. and 1-oz. wad slugs, and they are loaded in a traditional shotshell wad intended for variously loading among 7/8-oz., 1-oz., or 1-1/8-oz. shotshells. 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.

Published load recipes for wad slugs are available on the Hodgdon website under shotshell reloading, as well as from Lee Precision, who additionally sells moulds for casting drive key slugs from pure lead.

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 saboted 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.

Saboted slugs
Saboted slugs are lead-cored, full copper-jacketed or solid copper projectiles supported by a plastic sabot, which is 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). They can take the usual variety of shapes, but for maximum accuracy are typically bullet-shaped (ogive). The slugs are generally significantly smaller than the bore diameter, increasing the ballistic coefficient, and use the sabot to seal the bore and keep the slug centered in the bore while it rotates with the rifling. Saboted slugs, when fired out of a rifled barrel, are generally far more accurate than non-jacketed slugs out of a smoothbore, with accuracy to 300 m approaching that of low-velocity rifle calibers. By 1966 (published in 1966 Gun Digest), Ballistic Research Industries was producing a 440 gr .50 caliber sabot slug of a new design. The projectile was a wasp waisted hourglass shape made of a hard lead alloy. While many equate sabot to rifled barrel, this slug design was specifically designed for and tested in smoothbores. Testing even included over/unders and side-by-sides. The same slug, made of zinc and about 260 grains was marketed to police for car body penetration. The self-stabilizing shape allowed it to be used with bore cylinder or improved cylinder barrels.

Sabot variations
 * BRI
 * Brenneke Rubin
 * Gualandi / Palla Gualbo

Plumbata slugs
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).

Plumbata variations
 * Brenneke plumbata
 * Dangerous game (a.k.a. Gualandi boar)
 * Impact Discarding Sabot (IDS)

Steel slugs
There are some types of all-steel subcaliber slugs supported by a 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.

Guns for use with slugs
Many hunters hunt with shotgun slugs where rifle usage is not allowed, or as a way of saving the cost of a rifle by getting additional use out of their shotgun. A barrel for shooting slugs can require some special considerations. The biggest drawback of a rifled shotgun barrel is the inability to fire buckshot or birdshot accurately. While buckshot or birdshot will not rapidly damage the gun (it can wear the rifling of the barrel with long-term repeated use), the shot's spread increases nearly four-fold compared to a smoothbore, and pellets tend to form a ring-shaped pattern due to the pellets' tangential velocity moving them away from the bore line. In practical terms, the effective range of a rifled shotgun loaded with buckshot is limited to 10 yards or less. Iron sights or a low magnification telescopic sight are needed for accuracy, rather than the bead sight used with shot, and an open choke is best. Since most current production shotguns come equipped with sighting ribs and interchangeable choke tubes, converting a standard shotgun to a slug gun can be as simple as attaching clamp-on sights to the rib and switching to a skeet or cylinder choke tube. There are also rifled choke tubes of cylinder bore.

Many repeating shotguns have barrels that can easily be removed and replaced in under a minute with no tools, so many hunters simply use an additional barrel for shooting slugs. Slug barrels will generally be somewhat shorter, have rifle type sights or a base for a telescopic sight, and may be either rifled or smoothbore. Smoothbore shotgun barrels are quite a bit less expensive than rifled shotgun barrels, and Foster type slugs, as well as wad slugs, can work well up to 75 yards in a smoothbore barrel. For achieving accuracy at 100 yards and beyond, however, a dedicated rifled slug barrel usually provides significant advantages.

Another option is to use a rifled choke in a smoothbore barrel, at least for shotguns having a removable choke tube. Rifled chokes are considerably less expensive than a rifled shotgun barrel, and a smoothbore barrel paired with a rifled choke is often nearly as accurate as a rifled shotgun barrel dedicated for use with slugs. There are many options in selecting shotguns for use with slugs.

Improvements in slug performance have also led to some very specialized slug guns. The H&R Ultra Slug Hunter, for example, uses a heavy rifled barrel (see accurize) to obtain high accuracy from slugs.

Reloading shotgun slugs
Shotgun slugs are often hand loaded, primarily to save cost, but also to improve performance over that possible with commercially manufactured slug shells, which often cost over US$35 (2013) for a small box. In contrast, it is possible to reload slug shells with hand-cast lead slugs for less than $0.50 (2013) each. (The recurring cost depends heavily on which published recipe is used. Some published recipes for handloading 1 oz. 12 ga. slugs require as much as 49 grains of powder each, whereas other 12 ga. slug recipes for 7/8 oz. slugs require only 25 grains of powder.

Shotguns operate at much lower pressures than pistols and rifles, typically operating at pressures of 10,000 psi, or less, for 12 gauge shells, whereas rifles and pistols routinely are operated at pressures in excess of 35,000 psi, and sometimes upwards of 50,000 psi. The SAAMI maximum permitted pressure limit is only 11,500 psi for 12 gauge 2-3/4 inch shells, including slug shells, so the typical operating pressures for many shotgun shells are only slightly below the maximum permitted pressures allowed for safe ammunition. This small safety margin, and the possibility of pressure varying by over 4,000 psi with small changes in components, require great care and consistency in hand-loading.

Legal issues
Shotgun slugs are sometimes subject to specific regulation in many countries in the world. Legislation differs with each country.

The Netherlands
Large game (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, minimal caliber 5,56 mm and 980 joule at a 100 meters, and deer and wild boar, minimal caliber 6,5 mm and 2200 joule at 100 meters.

United Kingdom
Ammunition which contains no fewer than five projectiles, none of which exceed 0.36 inch (9mm) in diameter, is not controlled by law. 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—very strictly regulated—to possess. However slugs designed to expand, like all other expanding ammunition, are prohibited and require special permission such as for hunting.

United States
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 saboted slugs, which provides rifle-like power and accuracy at ranges over 150 yd.

Sweden
Slugs fired from a single-barrel shotgun are allowed for hunting wild boar, fallow deer and mouflon. 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. When hunting for wounded game there are no restrictions.