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Shotgun slugs, what are they and how do they work?
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.
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.
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 c 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 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.
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