Slug (projectile)

A slug is a term used for a solid ballistic projectile. It is "solid" in the sense of being composed of one piece; the shape can vary widely, including partially hollowed shapes. The term is occasionally applied to bullets (just the projectile, never the cartridge as a whole), but is most commonly applied to shotgun projectiles, to differentiate them from shotshells containing shot. Slugs are commonly fired from smoothbored barrels that are unable to impart the gyroscopic spin required for in-flight stability.

A water-slug refers to operating a submarine's torpedo tube that has been filled with water rather than a torpedo, thus shooting a "slug of water.".