...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. 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 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...