Is your upper and lower receiver jiggling? Are you having wobble troubles? You may have 99 problems but wobble aint one.... anymore, with this RetroArms Anti-Wobble Pin. 
The shaft of the pin is three cylindrical segments separated by angled edges all wrapping around a central screw. The pin slides into the receiver as normal but when the screw (hex / allen) is tightened the central segment is forced out of alignment sideways which locks the pin inside the receiver (until the screw is loosened).