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 functions a lot differently compaired the long barrel from Call of Duty: Black Ops II.
The long barrel change every weapon category different. The Submachine Gun tier will get a 25% increase in their maximum range. The Kuda for example has a range of 2.5 to 38 meters. What Long Barrel does is taking that 38 meters and bump that up by 25%. Making the new range 2.5 to 47.5 meters. The Shotgun tier will only get a 13% increase in their maximum range thus it won't help you get more one shot kills. The Handgun tier will get a 13% increase to all ranges. And Long Barrel doubles the range of Assault Rifle. So it's extremely effective on any Assault Rifle. Aesthetically, it adds an extension to a weapon's muzzle. It is now compatible with the Suppressor barrel attachment.