The ATPIAL Laser Sight is described in the game as:

A rail-mounted laser sight. Visible in NV.

You’ll find this in central Pucara province, at the Santa Muerte Tomb. A handful of cartel are here. The accessory case is on a crate in a large open shed.

This tomb is located at a hairpin bend in the nearby road. I parachuted in from a helicopter and landed on the tomb, but I recommend approaching from the rocky cliff that overlooks the area. I could only see three cartel when I was here, so they were very easy to take out.

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 ARs (found in San Mateo) 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.