The humble original M16 was originally Armalite’s AR-15, and was first ordered for military service with a contract issued to Colt Firearms in May 1962 for the purchase of early Model 01 rifles to be used by Air Force Security Police.
Fast forward to the XM16E1, which became the M16A1 in 1967, and you started to come closer to the standard Army/Marine rifle used in Vietnam and throughout the 1970s and early 1980s. It used a forward assist and a 1:12 twist barrel.
By 1983, the M16A2 came about, it had a thicker barrel in front of the front sight, a modified flash suppressor (closed on bottom), a new polymer buttstock (lighter and stronger), faster barrel twist (from 1:12 to 1:7), and a spent case deflector for left-hand users. Considered downright vintage by the Army and Marines, the Navy still sports them these days.
By 1998, the M16A4 was in play, primarily for the Marines, which had a removable carry handle, a Picatinny top rail to allow for optics, short rails on the handguard for accessories, and a 20-inch barrel with a 1:7 RH twist rate.
Since the GWOT kicked off in 2002, the big shift over the years has been to move from the full-length M16 family to the more compact M4/M4A1 carbine, with its collapsible rear stock and stubby 14-inch barrel, leaving the increasingly old-school style rifle as something of a relic today. Heck, the Army for the past couple years has been very actively working on replacing their 5.56 NATO rifles and SAWs with a new 6.8mm weapon.
Now jump to 2020, and the M16A4 is now apparently the Army’s designated rifle for Foreign Military Sales to equip overseas allies in places like Afghanistan, Iraq, Lebanon, and Nepal.
Colt and FN are competing in a contract to supply as much as $383 million smackers worth of M16A4s by 2025.
More in my column at Guns.com.