Plasma rifle

Plasma rifles or plasma casters 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, in the most common models, stabilized by electromagnetic claws upon exiting it.

Initially, the weapon was a bulky, industrial plasma caster, manufactured by Winchester Arms, however, later developments by the Enclave would lead to a more compact design.

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
 The Winchester Model P94 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.

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). 

P94 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 2277. 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 Enclave Plasma Rifle. The favourite weapon of Rivet City Security Chief Harkness, it seems to have been collected by the Commonwealth, and modified from there.

Vault-Tec Plasma Rifle
 A Plasma Rifle model found only in Vault-Tec Secret Vault.

Vault-Tec Turbo Plasma Rifle
 A unique Plasma Rifle variant found only in Vault-Tec Secret Vault.

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