There were a few carbines in the M16A2 series. They were the Model 723, Model 733, and later, the Model 727. The Model 727 is the only one of the three that had an A2 upper receiver. The Model 723 and 733 were based on the M16A1 to begin with. They used the same upper and lower. They transitioned into using the M16A1E1 (C7) upper as this was part of the M16A2 development. The later Model 723 was much like the later Model 727 in that the only real notable difference was the upper receiver.
Compare.
Late 723
727
The Model 723s used 1/7 twist barrels. The earlier ones used A1 profile (.625") 14.5" barrels and the later used the 1/7 twist Gov't profile (.750") with M203 cuts. The Model 733 Commando originally used a 11.5" 1/7 twist A1 profile barrel and transitioned to a 11.5" 1/7 twist Gov't profile (.750") barrel. The late version also had A2 upper and lower receivers.
Early 723
Late 733
I hope this helped a little bit. With Colt's production of the various models, variations were abound. Many times, it is hard to nail down just one version.
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