info from-
http://en.wikipedia.org/wiki/Shotgun_slug Brenneke Slugs 
A 12 gauge Brenneke 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 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 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 it does accuracy similar to that of American Foster slugs while retaining the improved penetration and slug integrity of the Brenneke design.
[edit] Foster Slugs
A
Foster slug, invented by Karl Foster in 1931, is a type of
shotgun slug designed to be fired through a
smoothbore shotgun barrel. The standard American domestic shotgun slug, they are sometimes referred to as "American slugs" to differentiate them from the standard "European slug" design popularized 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 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 through smoothbore barrels out to ranges of about 75 yards (69 m). Most modern Foster slugs also have "
rifling", which consists of thin fins on the outside of the slug. Contrary to popular belief, these fins actually impart no spin onto the slug as it travels through the air. The actual purpose of the fins is to minimize the friction on both the barrel and projectile and allow the slug to safely be
swaged down when fired through a choke, although accuracy will suffer and choke wear will be progressively accelerated when fired through any choke gauge tighter than open.
It is also possible to fire Foster slugs through rifled
slug barrels, though lead fouling (build-up in the rifle grooves) can be a problem. Accuracy is otherwise not appreciably affected in standard shotgun riflings.
[edit] Saboted slugs Saboted slugs are lead-cored, full copper-jacketed 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
give. 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 metres (330 yd) approaching that of low-velocity rifle calibers.
[edit] Steel slugs
There are some types of all-steel subcaliber slugs supported by a plastic sabot (otherwise projectile would damage the barrel). 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, said slugs are well suitable for shooting in brushes, but may produce
overpenetration. They also may be used for disabling vehicles by firing in the engine compartment or for defeating hard body armor.
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