The X277 "VIPER" magnetic rail cannon was developed for the U.S. Military by West-Tek. Also commonly refered 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, D.C. Museum of Technology in 2077.
This weapon is only mentioned in Fallout 3.