Colt AR15/M16 Lower Receiver Offset Adaptor PinAdapts Large Pin Colt lower receivers to fit standard small pin upper receivers. On the Colt AR15/M16, the front pin that joins the upper receiver to the lower receiver is larger than the pin hole on the popular new standard AR15 upper receivers. This is the pin the barreled upper hinges on when disassembling the rifle. Our Offset Adaptor Pin has a reduced diameter center section that is also off center. This allows a modern small pin upper to fit onto a Colt large pin lower. The offset center pin section allows positioning for the tightest fit. The two piece construction of this adaptor pin makes installation easy. Installation Procedure: Unscrew the removable end of the Adaptor Pin. Position your standard upper for installation onto your Colt lower. Push the small end of the Adaptor pin thru the lower and into the tighter fitting hole of the upper. At the same time, fit the larger end into the large hole of the lower. Now go to the other side of your rifle and screw the removable end of the pin thru the left large pin hole of the lower... Screw it onto the threaded end that is protruding from the small hole of the upper. With the end screw on but not yet tight, swing the upper assembly down into assembled position. Finally use a screw driver to twist the adaptor pin to take out any looseness of fit.