|*Don't get "screwed" in the "Butt", People!*|
The M16A1 and civilian Colt SP1 had a buttstock attached to the end of the buffer tube with a º-28 flat head machine screw (Colt part# SP92601). Then came the M16A2 in the 1980s. The new buttstock screw was longer (Colt part # SP64577). The new A2 stock was 5/8″ longer than the A1, although the same buffer tube is supplied for both models so a longer upper buttplate screw was required to engage the threads in the end of the tube. A1 screws run about 5/8″ or less and A2 screws are over 1 inch in overall length.
An A1 screw is not long enough to install an A2 stock. The threads won’t engage. Problems arise when an A2 screw is used to install an A1 or some aftermarket stocks. The end of the long screw will actually protrude into the buffer tube. This is bad... “long screw syndrome”...