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 changes every weapon category differently:
All assault rifles except KN-44 and KVK 99m have their range doubled
KN-44's range is increased by 13% and KVK 99m's range is increased by 30%
Submachine guns have their minimum damage range increased by 25%
Shotguns have their minimum damage range increased by 13% and minimum base damage decreased by 20% (meaning it does not help with one-shot kills, and allows them to reach further with decreased damage)
Handguns have their range increased by 13%
Aesthetically, it adds an extension to a weapon's muzzle. It is now compatible with the Suppressor barrel attachment.