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Foster slug
From Wikipedia, the free encyclopedia
A Foster slug, invented by Karl Foster in 1931, is a type of shotgun slug designed to be fired through a smoothbore shotgun barrel. Foster slugs can also be fired through rifled barrels, though lead fouling (build-up in the rifle grooves) can be a problem. 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 out to ranges of about 75 yards (about 70 meters). Most 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. Since the slug is fired at a supersonic velocity, the nose of the slug pushing a shockwave creates a vacuum on the side of the slug, where the fins are located. The actual purpose of the fins is to allow the slug to safely be swaged down when fired through a choke, although accuracy will suffer when such a slug is fired through tighter chokes.