Railgun

Railguns, also known as rail guns or rail cannons, are weapons seen throughout the Fallout series.

X277 "Viper" Magnetic Rail Cannon
Developed for the United States Armed Forces by West Tek, the only prototype was slated for display at the Museum of Technology from August 14 to December 31, 2077. It functioned by using energy cells to propel a depleted uranium round across a series of magnets. Mass producing the weapon was cost-prohibitive, and the project ended in 2044. 

Behind the scenes

 * In Fallout: New Vegas, an unused script in the game files for the Miss Fortune perk mentions both a light and heavy rail cannon.
 * A weapon called the 15mm ARTEMIS rail gun was planned for the cancelled game Van Buren.