Quoted:
Eyes aren't what they used to be. Can i put a a2 aperture on my a1 upper receiver?
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Not only can you as several others have already said, it is how my issued rifle was configured.
Before the M4 and while the M16A2's were standard issue, I was issued what we called CAR-15's - 14.5" lightweight barrels on A1 uppers, lowers were SAFE, SEMI, and AUTO with two position aluminum collapsible stocks. Since the M16A2's were in service and parts were available, it was an easy replacement to install the A2's rear sight aperture. I felt it provided a better sight picture.