06-19-2012, 07:45 PM
Colt Started that practice with the civilian SP-1 in 1963, so that they wouldn't be compatible with M16 uppers. The SP-1 upper also does not have a machined recess for the AUTO sear to articulate in underneath the upper by the takedown pin lug. If you hold an SP-1 upper next to any other AR15 upper, you'll see the difference, though it makes no relevance to those of us who don't own registered machineguns.