Described in the game as:

Uses three lasers for fast target acquisition

It’s located in the centre of San Mateo province, inside a cartel boot camp. A “Gasoline Drop” side mission to steal a helicopter is nearby. The accessory case is inside the armoury in the middle of the camp.

After taking out the snipers in the towers, you can take a wide approach to the camp around the back on the southern side, take out the sicarios guarding the helicopter side mission and walk in from the shooting range. The tricky part is all the trainees in the middle of the camp, so I set a C4 charge on the generator to the west and set it off as a distraction to draw them away from the armoury.

The three lasers are meant to improve accuracy, but I do find them slightly distracting when you have obstacles in the foreground. I guess it makes you look a bit like the Predator though, which could be cool. But I do equip this on my assault rifle loadout.