OK. This will be a brief, and hopefully, clear genesis of the AR-15/M16 family.
The first AR-15 (AR = Armalite Rifle) was a select-fire assault rifle manufactured by Armalite, Inc. They had the 3-prong flash suppressor, green plastic furniture, chrome-plated bolt assemblies, and no forward-assist. Because Armalite was not able to do volume production of the rifle, Colt's Firearms became the licensed producer of the rifle.
Because the original AR-15 was select fire, Colt's offered a semi-auto only version called the Colt Sporter or SP1. These rifles were also called AR-15's. The first civilian-legal AR-15 Sporters became available in 1964. They had modified chrome-plated bolts, 3-prong suppressors, black furniture, and no forward assist. The bottom of the bolt carrier, hammer, disconnector, trigger, and selector were redesigned for semi-auto fire. To keep an M16 upper from being fitted to an AR-15 lower, Colt adopted a two-headed screw to replace the front push-pin of the M16 lower. Colt produced rifles retain this feature to this day. All other commercial AR-15 semi-automatic rifles and carbines have reverted to the front and rear captive push-pin configuration.
The American Department of Defense adopted the select-fire AR-15 as the M16. These rifles had 3-prong suppressors, black plastic furniture, and no forward assist. The war in Vietnam heated up and improvements were made to the design.
A specialized, short version of the standard M16 rifle was designed. It was called the CAR-15 and used a shortened, sliding rifle butt stock, modified triangular handguards, and a special "sound and flash" suppressor on its 10-inch barrel. The CAR-15 (Colt Automatic Rifle) was redesigned into the XM177 5.56 NATO submachine gun. The improved XM177 uses an improved sliding butt stock (similar to today's M4 carbine) and redesigned tubular handguards. The barrel remains at 10-inches and it does not have a forward assist. The XM177E1 soon follows the XM177 on the production line; the only difference is the barrel grows to 11.5 inches to improve reliability.
In the 1967-1968 period, the M16 is modified to the M16E1 (later M16A1). The major recognition points are a closed ("bird cage") flash suppressor and forward assist mechanism for the bolt. A 30-round magazine is also introduced that eventually replaces the standard 20-round.
The XM177E2 submachine gun is also introduced at the same time. The 'E2 retains the 11.5-inch barrel but adds the forward assist feature of the M16E1.
The Colt AR-15 undergoes several modifications during the late 1970's that make it different from its military cousins. Modifications: (1) trigger and hammer pins are increased in size so that the smaller pins used in earlier AR-15's and M16's will not work, (2) the 3-prong flash suppressor is replaced by the closed suppressor, (3) the bolt carrier and uppre receiver are modified to the M16A1 standard with the forward assist function, and (4) the underside cutout on the bottom of the bolt carrier is deepend until the underhang is completely eliminated.
In 1982, the USMC adopts the product-improved M16A2 rifle. This rifle introduces the symetrical round handguards, flash suppressor/compensator, heavier 1 in 7 twist barrel, fired case deflector (to accomodate left hand shooters), and 3-round burst feature (replaces the full-auto). Shortly after, Colt's releases the AR-15A2 that has the same features as the M16A2, except for the 3-round burst feature.
Because special operations forces need an equivalent to the XM177-series weapons that are now well-worn, the M16A2 is redesigned into the M4-series 5.56 NATO Carbine. The M4 uses an improved telescoping stock of the XM177, it uses the 'A2 rear sight, case defelector, shortened/round handguards, and a 14.5-inch 1 in 7 twist barrel, contoured for a M203 grenade launcher attachment, and the flash suppressor/compensator of the 'A2 rifle.
Beginning in the 1970's and continuing, other manufacturers began making lower receivers so that surplus military parts could be reutilized. These new "AR-15's" had all the features of the military versions and eliminated the peculiar features of the Colt products. Frist to go was the Colt-specific two headed screw; the new receivers reverted to the front push-pin. The Colt oversize trigger and hammer pins were not adopted, meaning standard military pins could be used. Military improvements to the lower receiver, reinforced front push-pin lugs, beefed-up rear receiver about buffer tube, and raised guard around the magazine release were added. Colt was slow to add these features until the AR-15A2 came on the market.
During the late 1980's and 1990's the AR-series of rifles offered and variants explodes into the bewildering array that we see today. It seems like even more variations are unveiled every few months.
And that's the rest of the story.
