Short history on the XM177/M4
With some exceptions the program started with the XM177 series in 1966 then progressed to the following course:
XM177E2/USAF GAU/5 1967/68-1975/76. Colt stopped making them because of problems with the short barrel and moderator combo. They were used up untill Desert Storm(USAF and State Dept. personnel)
Colt 653 1975/76-1985 Navy(USMC), Army, USAF, and Federal Govt. purchased several thousand during that time period. Most still in use have been updated to A2 specs or rebarreled(USAF).
Colt 723/XM4 1985-1988 Navy and Army. Basically the 653 with a 1/7 14.5" barrel.Has A1 sights, some have case deflector. Have all been updated to A2 specs. Large number were purchased for SF use during Panama(89)
Colt 727/M4 1988-1994 Navy, Army and Some USAF. This was the first A2 carbine that the US DOD purchased. Has been replaced by the M4
Bushmaster M4 1990-1991 Bushmaster made a short run of M4's for the DOD. Hard to spot. None are stock Bushmaster. All furniture was replaced with Colt parts before fielding. First 150 or so had Colt Alum. Stocks.
Colt 927/M4/M4A1 1994-Present. Current issue/Spec. Most older Carbines have been up-graded to this spec.
This is just a synopsis.