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 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/
This mod is intended to add the Service Rifle from New Vegas to Fallout 3. It is added to the level list for Talon Company, Super Mutants and the Smugglers from Point Lookout. It will conflict with other mods that do the same unless they add theirs via script. I do not know how to add weapons to the lists with a script, but if anyone else wants to do so, feel free to do so and upload it.
Required Files
meshes/weapons/2handautomatic
servicerifle.nif
textures/weapons/2handautomatic
battlerifle.dds
battlerifle_m.dds
battlerifle_n.dds
textures/interface/icons/pipboyimages/weapons
weapons_battle_rifle.dds
sounds/fx/wpn/riflevarmint/fire_2d
wpn_varmintriflefire2d_02.wav
sounds/fx/wpn/riflevarmint/fire_3d
wpn_varmintriflefire3d_04.wav
sounds/fx/wpn/riflevarmint/fire_3ddist
wpn_varmintriflefire3ddist_05
Place the files in the files in the following locations in your Fallout 3 Data Folder and replace the blank placeholder file.
meshes/Service Rifle
servicerifle.nif
textures/Service Rifle/weapon
battlerifle.dds
battlerifle_m.dds
battlerifle_n.dds
textures/Service Rifle/icon
weapons_battle_rifle.dds
sound/Service Rifle/fx/2D
wpn_varmintriflefire2d_02.wav
sound/Service Rifle/fx/2D
wpn_varmintriflefire3d_04.wav
sound/Service Rifle/fx/2D
wpn_varmintriflefire3ddist_05
Known Issue
There is slight clipping for female characters when unholstered, see image 3 for example.
I am uncertain on how to fix it, if anyone wants to fix and upload a patch, feel free.