REQUIRES FALLOUT NEW VEGAS!!!
If you do not own, I suggest getting it from GOG.com but Steam should work just fine. You must extract the bsa files named Fallout - Meshes, Fallout - Textures and Fallout - Sounds and GunRunnersArsenal - Main, all of which can be found in the Data folder for New Vegas. I recommend using B.A.E. 
https://www.nexusmods.com/fallout4/mods/78/

Required Files
meshes\weapons\2handrifle
multiplasrifle.nif
multiplasrifle02.nif

sound\fx\wpn\riflemultiplas\fire_2d
wpn_multiplasrifle_fire_2d_01.wav
sound\fx\wpn\riflemultiplas\fire_3d
wpn_multiplasrifle_fire_3d_01.wav

textures\weapons\2handrifle
multiplasrifle.dds
multiplasrifle_g.dds
multiplasrifle_m.dds
multiplasrifle_n.dds
multiplasrifle02.dds
multiplasrifle02_g.dds

textures\interface\icons\pipboyimages\weapons
weapons_multiplas_rifle.dds

Place the files in the files in the following locations in your Fallout 3 Data Folder and replace the blank placeholder file.

meshes\weapons\2handrifle
multiplasrifle.nif
multiplasrifle02.nif

sound\fx\wpn\riflemultiplas\fire_2d
wpn_multiplasrifle_fire_2d_01.wav
sound\fx\wpn\riflemultiplas\fire_3d
wpn_multiplasrifle_fire_3d_01.wav

textures\weapons\2handrifle
multiplasrifle.dds
multiplasrifle_g.dds
multiplasrifle_m.dds
multiplasrifle_n.dds
multiplasrifle02.dds
multiplasrifle02_g.dds

textures\interface\icons\pipboyimages\weapons
weapons_multiplas_rifle.dds

How to Find
The Multiplas Rifle has a chance to appear on Enclave Soldiers that would usually be using Plasma Rifles.

Compatability
Outcast Collection Agent Expanded - One of the main files allow for you to give the Plasma Caster to Protector Casdin.
Plasma Defender - If you want to give both the Plasma Defender and the Multiplas Rifle to Casdin, use the patch for the Standalone Main File.
P94 Plasma Caster - A compatability patch for this has been made for the Standalone file. There is also a patch if you use both the P94 Plasma Caster and the Plasma Defender.

Known Issue
Texture for the 1st Person view is a bit low quality.