The ATPIAL Laser Sight is described in the game as:
A rail-mounted laser sight. Visible in NV.
It can be found in southern Barvechos, at a motel just off the main road in and out of the province. A “Sicario Leader” side mission is nearby. The accessory case is sitting on a fountain in the centre of the motel parking lot.
I left my car across the street from the motel while I popped up my drone. There are five sicarios here, plus the sicario leader for the side mission. If you want to do the side mission, it’ll have to be done before you pick up the accessory, or you’ll have to come back again later to try it again.
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 handguns (found in Montuyoc) 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.