This is described in the game as:
Reduces recoil. It is incompatible with suppressors.
It’s located in southern Remanzo, inside a cartel controlled energy drink factory. The “Energy Drinks” mission is nearby. The accessory case is upstairs inside a building in the middle of the factory complex.
I wasn’t prepared for this approach at all and it was a mess! I went in guns blazing and received support from passing rebels, but the cartel called in a helicopter and it wasn’t pretty. If you wanted to keep a low profile, you could sneak in the entrance and work your way to the building. There are stairs inside the building to take you up to the accessory case.
This is the V2 version of the compensator muzzle for all sniper rifles and reduces recoil. However it’s not compatible with the suppressor, so you can’t equip the suppressor with this accessory on your rifle. Personally, I prefer a suppressor every time, especially with the ADV Suppressor skill unlocked.