Described in the game as:
A rail-mounted laser sight. Visible in NV.
You can pick this up in central Mojocoyo province, in a cave under a hill, off the adjacent road. The cave has two sicarios outside and four inside with a mounted gun. A “Parachute Drop Site” side mission is nearby. The accessory case is at the very back of the cave, behind the drop site supplies.
I didn’t realise where this accessory case was – I was in a helicopter and flew straight to the icon on the map but couldn’t see it. Then when I landed on the road to check for a cave, the cartel opened fire and it was a mess. I recommend approaching from the road and taking your time to kill the two lookouts at the front first, before sync-shotting the ones inside, keeping them away from the mounted gun.
The ATPIAL laser sight is the same for all gun classes. It offers increased accuracy while sacrificing some handling because it adds weight to the front of your gun.
On the negative side, it’s beaten in the stats by the Laser 3Dot for sniper rifles, found in Flor de Oro, which offers more accuracy and less of a handling penalty. On the positive side, the ATPIAL is (in my opinion) more helpful and less of a distraction when in use.
I suggest you pick up and try both to decide for yourself.