The long barrel is described in the game as:
Increases muzzle velocity.
This can be found in eastern Itacua, at the arse end of the province in Khochi Town. The accessory case is upstairs inside a warehouse at the eastern edge of the town.
There are a lot of sicarios around, but as usual, being Itacua it’s not exactly difficult. I first freed some caged rebels to use as a distraction, then worked my way clockwise around the perimeter of the town to the warehouse, taking out any enemies directly in my way. From that edge of town you can get directly to the warehouse with minimal fuss.
The long barrel for ARs increases damage and range, and to a lesser extent, accuracy. For this, there’s a slight handling penalty. Whether you equip this or not will definitely depend on which AR you use, and how you want it to perform for you.
Personally, I use the ACR with the M203 grenade launcher which hampers handling as well. Because the ACR is well balanced for damage and range already, I actually equip the short barrel (found in Agua Verde) to bring the handling back up. But this will be totally up to you and very much a personal choice.
Maybe pick this up so you can experiment with different ARs and tweak their stats to your liking.