"Slug-thrower" or "slugthrower" is a colloquialism referring to projectile weapons that expel a slug or bullet (usually of metal) at high velocity. The most common form of such weapons are chemical firearms, which use the pressure wave of a small chemical explosion to accelerate a bullet within a confined barrel. Other forms of these devices accelerate the slug by other methods, such as compressed gas, linear magnetism, micro-rockets, and so forth.
Slug-throwers were considered a "low-tech" and less destructive weapon, at least to buildings.
Slug-throwers were used by the Orion civilization in their distant past. The first Orion slaves were armed by Sugg slavers to defend themselves against rival slavers. Later, small primitive slug-throwers were used in a bloody genocidal civil war on the Orion Colony of Votannis, which killed over 40 million people but left buildings mostly intact. (FASA RPG module: The Orions: Book of Common Knowledge)