Well so far my AP4 is stock except that I bought a Burris AR-332 and mounted it on the carry handle. To do this you must remove the mounting base that comes with the AR-332 and mount it on the carry handle. You will have to drill an additional hole on the top of the handle and then drill both of the holes on through the base of the handle and then go back and slightly enlarge the holes through the base to enable the use of a screwdriver so it can be mounted. When mounted you will notice a hole through the base which will allow you to use the iron sights which are on the rifle.