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There is nothing different in a mil-issued upper versus the uppers that are on the market for the last 20 years. If it's an actual mil upper it's going to be 14.5 so would be pistol or NFA territory on barrel length. If it's 16 its just a random AR15 upper.
At one time Colt made some uppers and bolt carriers that would not work on an FA lower, but it doesn't really mean anything, there is nothing an upper can do to make something be FA.
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The upper for an M16 has a relief cut for the sear to clear
Attached FileThis changes nothing functional or legal wise on the upper on a semi lower
Colt made LOWERS and bolt carriers that would not work with full auto parts. The lowers had a sear block that wouldn’t allow for sear placement, drop in auto sears or lightning links. It was a chunk of metal in the lower
Attached FileThe bolts were “half moon” ones to clear the sear block and prevent the use with any full auto parts. They also had unshrouded firing pins
Attached FileAnd OP is talking M16 spec which would be a 20” pencil profile barrel