Plasma rifle

Plasma rifles are high tech weapons firing superheated bolts of plasma, powered by Micro Fusion Cells, which found applications in military and industrial fields. The bolt forms in a special chamber, which is then sent down a superconducting barrel and stabilized by electromagnetic claws upon exiting it (if the claws are present in the model at all).

Regardless, all types of plasma rifles have a common feature - when a critical hit is delivered, they can literally melt down enemies. While industrial casters weren't powerful enough to dissolve bones too, later ones had no problem converting the target into a (literally) puddle of goo.

Winchester P94 Industrial Plasma Rifle
 The Winchester Model P94 Plasma Rifle is a bulky, industrial-grade energy caster, firing superheated bolts of plasma down a superconducting barrel ending in electromagnetic claws helping to form the plasma bolt.

Winchester P94 Military Plasma Rifle
 In the military version, the company has downsized the weapon and revamped its design, making it a man-portable energy rifle, complete with a stock, pistol grip and carry handle (which can, presumably, also mount optics). 

Turbo Plasma Rifle
 The Turbo Plasma Rifle is a modified Winchester P94 Plasma Rifle. The plasma bolt chamber has been hotwired to accelerate the bolt formation process, resulting in an increase in the rate of fire 

Enclave Plasma Rifle
 An interesting model found in the hands of the Enclave on the East Coast, around 2077. It is similar to the Enclave's Enclave Plasma Pistol, and, unlike the P94, does not have a superconducting barrel nor the electromagnetic claws - just the plasma bolt chamber and a discharge coil. 

A3-21's Plasma Rifle
 A3-21's Plasma Rifle is an upgraded version of the normal Plasma Rifle. It is one of the strongest weapons in the game, and can be obtained very early on. This weapon can also kill a sentry bot, deathclaw, or any strong monster of a sort, with about 4 or 5 shots.

Appearances
Plasma rifles, as described in detail above, appear in all but one of the Fallout games.