Frankenstein Assault Rifle
It's a cobbled-together gun made from random other guns. It uses 7.62mm x 51mm rounds.
This can't be found anywhere. I made it up, you see.
You can add a flashlight, front grip, and Hello Kitty paint job. Despite the name of the Hello Kitty paint job mod, it lacks any Hello Kitty logos or emblems.
The Frankenstein Assault Rifle doesn't appear in Fallout: New Vegas.