Many shotgun slugs are designed to be stable when fired from a smoothbore barrel, which lacksthe rifling normally used tostabilizethe projectile. The simplest shotgun slug is a round ball, often called a pumpkin ball or pumpkinshot. Since it is symmetricon all axis, the round ball will not significantly deviate from its path if ittumbles. A shotgun firing a round ball is similar inperformance toa smoothbore musket, and the restriction of the spherical shape limits thesectional density possiblefor a leadprojectileof a given bore diameter.
To obtain higher sectional density and better penetration, an elongated slug is needed, and a method must be provided to prevent the slug from tumbling. Foster slugs are designed with a deep cup in the back, so that the centre of mass is moved forward. The forward mass of the slug helps keep it stable, and will tend to keep the slug moving point first.
Many Foster slugs are also rifled. Rifled slugs have what looks like rifling cast into the surface, Contrary to popular belief the rifling does not provide any spin. These cast ridges allow the slug to be safely swaged down when fired through a choke.
A variation on the Foster design is the Brenneke slug, which uses a solid lead rifled projectile with an attached plastic, felt, or cellulose fibre wad that provides drag stabilization. Brenneke slugs are more suited for dangerous game, as the solid slug is less prone to deformation than the hollow Foster type.
Sabot slugs are generally designed to be fired from a special rifled shotgun barrel. Sabot slugs are smaller than the bore diameter, and offer significant advantages in external ballistics with the reduced drag. Some shotgun slugs also use fins or a lightweight plastic portion in the rear to provide stability from smooth bores, and may be designed to work with or without sabots.
Shotgun slugs intended for use in smoothbore barrels need to be made out of very soft lead alloys or have a compressible sabot, as they must be able to fit through the restrictive choke present in most shotgun barrels. Even so, it is not recommended to fire slugs through very constrictive chokes, as the effort of compressing the slug will at the least damage the end of the barrel effectively reducing the degree of choke, and at worst significantly raise the pressure within the barrel to cause a burst or explosion.
The Foster slug, invented by Karl Foster in 1931, is a type of shotgun slug designed to be fired through a smoothbore shotgun barrel. The defining characteristic of the Foster slug is the deep hollow in the rear, which places the centre of mass very near the tip of the slug, much like a shuttlecock. If the slug begins to tumble in flight, drag will tend to push the slug back into straight flight. This gives the Foster slug stability and allows for accurate shooting out to ranges of about 50 – 70 yards.
Foster slugs may also have rifling, which consists of eleven or twelve fins either cast or swaged on the outside of the slug. Contrary to popular belief these fins actually impart no spin on the slug as it travels through the air.
The actual purpose of the fins is to allow the slug to be safely swaged down when fired through a choked shotgun barrel, although accuracy will suffer when such a slug is fired through chokes tighter than improved cylinder, with a cylinder choke being recommended for best use.
As with all shotgun slugs it is possible to fire Foster slugs through rifled slug barrels, but if doing so leading of the rifling and barrel becomes a great problem necessitating regular cleaning to maintain any degree of accuracy.
The Brenneke slug is similar in appearance to a rifled Foster slug. The Brenneke slug was developed by the famous German gun and ammunition designer Wilhelm Brenneke (1865 – 1951) in 1898. The original Brenneke slug is a solid lead projectile with fins cast onto the outside, much like a modern rifled Foster slug. There is a plastic, felt or cellulose fibre 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 or rifling are easily deformed to pass through choked shotgun barrels. Extensive tests have shown these fins do not impart any stabilizing spin on the projectile.
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. The sharp shoulder and flat front of the Brenneke mean that its external ballistics restrict it to short range use, as it does not hold velocity well. The Brenneke slug in 12 gauge is well suited for large and dangerous game at close ranges, and deer sized game out to about 50 – 70 yards.
Brenneke slugs in the .410 calibre are useful with smaller game and deer, but usually at a much more reduced range of about 30 – 50 yards. Brenneke slugs are somewhat more accurate than the Foster slugs, but are usually more expensive.
The main characteristic of sabot slugs is the plastic carrier or sabot, which is of bore size or sometimes a little larger to enable the sabot to engage the rifling found in modern slug barrels. The slugs contained in sabots are usually of pistol calibre with hollow points. Although the sabot slug is used primarily in rifled barrels, some designs of sabot slugs can be of use in smoothbore shotguns most notably the Brenneke Rubin Sabot, a sub-calibre slug utilizing the familiar Brenneke attached wad system, and the “Palla Gualbo” again a slug using an attached wad system.
The smaller projectile held within sabots will have a much flatter trajectory, and will travel at much higher velocities than the more traditional foster or rifled slug, which coupled with a rifled slug barrel will increase accuracy and range to near rifle proportion. Another advantage of the sabot type of shotgun slug is no lead comes into contact with the barrel at all, so preventing lead fouling. Which is of course excellent for the slug shooter wishing to use his shotgun for target shooting as well as hunting.
Reprinted by permission courtesy http://www.buckandslug.co.uk/