Bottom line, Colt bought the AR15 from the original Armalite. They have the TDP for both the M16 and M4. Thats what they build. BCM and Daniel Defense and others will also built to mil spec, but Colt has it down.
Buy what you like. But you can't go wrong with Colt especially for what 6920s have been available for lately.
"Iwoke up this morning and got myself a beer. The future is uncertain and the end is always near."