Railgun

Railguns, also known as rail guns or rail cannons, are large weapons, using electromagnets to fire slugs at high velocity. There were multiple projects attempting to create successful designs before the Great War. A similar weapon which uses coiled electromagnets to propel a slug are called gauss rifles.

X277 "Viper" magnetic rail cannon
The X277 "Viper" magnetic rail cannon was developed for the U.S. Military by West-Tek. The weapon uses energy cells to propel a depleted uranium round across a series of magnets discharging it at extremely high velocity. Deemed too costly to produce on a mass scale, the project was abandoned in 2044. The only known prototype was displayed in the Museum of Technology in 2077, but is no longer present as of 2277.

15mm ARTEMIS rail gun


The 15mm ARTEMIS rail gun was an unfinished project by Poseidon Energy and the Enclave. Its schematics can be found at Mesa Verde and could possibly still be completed.

Behind the scenes
While not present in Fallout: New Vegas, game files suggest light rail cannons and heavy rail cannons may have at one point been intended to be in the game. Railguns were also intended to appear in J.E. Sawyer's Fallout Role-Playing Game.