The ATPIAL Laser Sight is described as:

A rail-mounted laser sight. Visible in NV.

You can collect this in southern Ocoro province, at Rebel Hideout Santa Rosa, a house set off a road. The accessory case is inside the house.

There are five enemies around the house, but they’re spaced out and there’s lots of foliage, vehicles and walls for cover, so they’re very easy to take out. You have to kick the door down on the east side of the house to gain entry.

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 SMGs (found in Tabacal) which offers more accuracy and offers 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.