M4A1, 1995
View attachment 5900170
M4 -> M4A1 conversion, 1994, note that serial number ranges are in blocks, so a high SN might be found on earlier issued weapons
View attachment 5900231
M4, 1987, Colt 6520 and 727
View attachment 5900171
XM4 (M16A2) 1986, Colt 725 and 727 (late)
View attachment 5900183
XM4 (M16A1) 1982, Colt 653
View attachment 5900176
there's also the Air Force GAU-5A (US Army XM177, later XM177E1 with forward assist, E2 with the new grenade ring for mounting the M203) and GUU-5P which were Colt 609 and 610, with the 609 literally being marked "AR-15 Commando", and the 610 having no forward assist (the 610B with a 4 position safe/semi/burst/auto FCG), but these aren't really "M4's" anymore and are closer to the original Commando concept.