The automatic barrel provides a better rate of fire, range and sighted accuracy in exchange for worse recoil, reduced ammo capacity and poor hip-fire accuracy.
The recipe for this weapon mod varies depending on the weapon for which it is made:
Fiber optics (2-5)
Automatic barrel (1)
Can be found and used at any weapons workbench, if the player character possesses the above materials.
While this mod increases the effective damage per second of weapons it can convert into an automatic, it also greatly increases the ammunition usage of the weapons.