The ACE Internal Receiver Block requires modification to the receiver; i.e.. the tang on the back of the receiver must be cut off then filed or ground smooth. The internal receiver block will then simply bolt into the receiver. It will be connected at the stock screw hole on the inside of the receiver and the pistol grip "T" nut will hold the front of the receiver block. There is no movement to the receiver block once it is installed. Fits most common stamped receivers, 1mm and 1.6mm.