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 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. 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.
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