This weapon is comprised of a rocket motor attached to a chainsaw, and is a special ammunition type available for the Missile Launcher.
This is a custom ammunition type created especially for the NIF-Bashed Weapons Mega-Pack.
I started with the base game Chainsaw mesh and stripped it down, removing the handles, pull cord, and all other extraneous bits. I then added the rocket motor of the ICBM from the Lonesome Road DLC, scaled down to size.
All weapons firing Missiles can also fire this ammunition.
This ammo type can be crafted at any reloading bench.