The ATPIAL is described as:
A rail-mounted laser sight. Visible in NV.
You’ll find it in northern Malca province, in Uma Marca town. The “Show Canceled” mission is nearby, as well as a “Comms Tool Air Transport” side mission to steal a plane. The accessory case is in the yard of a house at the north of the town, near the airfield.
This place is crawling with sicarios and has an alarm. You can approach relatively close from the main road that runs from the north, where a small path leads up near the house. Some narcos patrol the dirt road in front of the house, and there’s a cartel lieutenant out front as well.
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 LMGs (found in Mojocoyo) 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.