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 only increases the minimum damage range of a weapon, meaning if a weapon takes a maximum of 5 shots to kill, it only increases the 4-shot-kill range. Aesthetically, it adds an extension to a weapon's muzzle. It is now compatible with the Suppressor barrel attachment.