"Slug" is a general term for any unguided (or semi-guided) projectile. Slugthrowers are as old as our family: apes evolved with rocks in their hands. Mechanically-launched slugs appear in the Late Pleistocene to Holocene in the form of spearthrowers, slings and bows; complex mechanically-launched slugs were first developed by the Ancient Greeks and Chinese (ballistae and crossbows), and chemically-propelled slugs by the Medieval Chinese and Europeans (fire arrows and cannons).
Slugs dominated warfare from the 14th through 20th centuries. Cannon ruled sieges and large battlefields; muskets and rifles short-ranged combat. The subtype of slug called a shell – essentially a slug with an explosive and detonation mechanism – delivered by artillery tube or dropped from an airplane, ruled the battlefields of the 19th and 20th centuries. Even the first use of nuclear weapons, in World War II, was in the form of air-dropped bombs: nuclear missiles did not become a reality until the 1950’s and were not deployed in numbers until the 1960’s.
With the development of missiles, slugs and shells became increasingly less important in strategic warfare. Aside from personal combat, the main role of slugs and shells in 21st-century warfare was as defensive weapons designed to shoot down attacking missiles. These autocannon had an important combat role well into the 22nd century, though principally to backstop anti-missile and anti-torpedo defenses based primarily around lasers, blasters and missiles.
In the late 22nd century the development of the gausson (“gauss cannon,” basically an electromagnetic slugthrower) to full practicality gave the slugthrower a new lease on life. Gauss-cannon could throw their slugs much faster than chemical propulsion, and the impact of the projectiles at several, eventually dozens of miles per second delivered concentrated energy which was superior to anything producible by small energy weapons. The main limitation on the gauss-gun was range: even if it fired a self-guiding projectile (essentially a very small missile) it was still essentially a close defense system.
In the 24th century the leaper (“laser-propelled round”) further extended the range and speed of slugthrowers. A leaper was propelled primarily by a laser mounted on the launching platform, though it might be given a starting boost by a gausson or other mechanism. This allowed the leaper to accelerate all during its run, reaching terminal velocities of hundreds of miles per second, and it could use the received energy for delta vee, making it a sort of missile with a very limited engagement cone. Leapers were important because they pushed the offensive-defensive projectile equation strongly to the defense, which meant that torpedoes were greatly weakened and such weapons as meson cannon and neutron blasters greatly strengthened in space combat.
Finally, the late 26th century saw the development of the arkvee (“R-K-V” or “relativistic kill vehicle”) to practicality. This was essentially a very fast leaper, using antimatter-pumped lasers on the gun mount to allow acceleration to many thousands of miles per seconds. An arkvee, as the name implied, could if powerful enough reach a good fraction of “C” (the speed of light) and its kinetic energy would be immense: at such velocities fairly small slugs impacted with the force of contact nuclear explosives.
Arkvees very strongly threw the balance of force to the offensive, and put a premium on powerplant, agility and active shields as the principal defense against such weapons.. When coupled with the gravitic-harness inertial damper, which enabled crewed ships to maneuver at hundreds of gravities, arkvees very much weakened all fixed defenses, such as the space fortresses which the Late Mandate had so very much relied upon to defend itself against the Outies. The use of large numbers of small antimatter-powered, photon-driven grav-harnessed warships, armed with arkvees and disruptor cannon, by the Hordes which swept in from the Outer System during the 27th and 28th centuries, were the immediate cause of the fall of the American Mandate in Space, and its reduction to a remnant in Eurasia for the next millennium.