The Long Barrel increases the range of a weapon by 16%. Therefore, the longer the initial range of a gun is, the longer the maximum range will be. The attachment is available for submachine guns, shotguns and handguns. It is incompatible with the suppressor attachment, for balancing reasons.
The Long Barrel appears as an attachment for several weapons in Call of Duty: Black Ops III. It increases the effective range of a weapon. Aesthetically, it adds an extension to a weapon's muzzle. It is now compatible with the Suppressor barrel attachment.