The original Colt scope was used from Vietnam to Operation Iraqi Freedom
Magnified optics on standard infantry weapons was not a common practice until the Global War on Terror with the adoption of the ACOG. The Marine Corps, with its emphasis on long-range marksmanship, made the 4x scope general issue while the Army issued it to soldiers in certain leadership positions. Before the ACOG, another magnified optic could be found on American infantry rifles: the Colt Scope.
Colt's sale of the AR-15 as the M16 to the U.S. military represented a huge change in American small arms. The weapon's lightweight construction and intermediate cartridge gave troops in Vietnam a much better fighting tool compared to the heavy M14 and its full-power rifle round. To increase the capability of the AR-15, Colt introduced the Colt Telescopic Sight.
Also known as the Colt Sporter Scope, the magnified optic was not designed specifically for military use. With an integrated mount and clamp, the scope attached directly to the AR-15's carry handle without any need for tools. Two versions of the scope were made: one with 3x magnification and another with 4x. The first versions were made for Colt by Realist in the United States. Later, from the 1970s-1990s, it was made by Hakko Optics in Japan.
The Colt scope used a fine duplex reticle paired with a ballistic-drop-compensated elevation turret. Once the optic was zeroed at 100 yards, the reticle could be adjusted to account for bullet drop at longer ranges. It was calibrated for the velocity and weight of the standard military 55-grain .223 bullet from a 20-inch barrel. The literature included with the scope noted that it "requires practically no maintenance" and is sealed to be "completely weather-proof." Still, in the event that the optic became damaged and unusable, the rifle's iron sights could still be used through the scope mount.
The Colt scopes found their way over to Vietnam and were used on M16 rifles and CAR-15 carbines. Used primarily by more specialized units like MACV-SOG, these scopes were likely privately purchased or bought with unit funds. At any rate, conventional troops weren't being issued the optics from their arms rooms. Colt continued to sell Sporter Scopes until the 1990s, but it and Colt-style copies were used through Operation Iraqi Freedom. The Colt scope was a good fit for the M16A2, which retained the fixed carry handle and was even seen mounted on the removable carry handle of the later M16A4.
Today, original Colt scopes are highly sought after by collectors and vintage rifle cloners. To meet this demand, the retailer Brownells contracted Hakko in Japan to recreate the Colt scope. The Brownells-branded replica even features the original yellow inspection sticker on the bottom of the tube.