X277 "Viper" Magnetic Rail Cannon

The X277 "Viper" Magnetic Rail Cannon was developed for the U.S. Military by West-Tek. Also commonly referred to as a "Railgun", 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 Washington, DC Museum of Technology in 2077.

Behind the scenes
It could be assumed that the U.S. Military used the data once the project had ended to downscale this device into the more portable and financially feasible M72 Gauss Rifle.

Appearances
This weapon is only mentioned in Fallout 3.