USMC forces were the first to adopted the A2 models and they were then followed by the Army in 1986. The model introduced a more effective 5.56mm cartridge and added a new, adjustable rear sighting device. Other changes included a heavier barrel assembly, revised pistol grip and buttstock units, and a case deflector fitted near the ejection port. A semi-automatic and thee-round burst firing function were also part of this new variant and the rifle managed an extensive combat service record all its own and continued to solidify the M16 line as a proper frontline combat weapon.
The "M16A3" was a limited-production version based on the M16A2 while featuring the trigger group of the M16A1. This model was mainly used by specialist forces including the U.S. Navy SEALs.
In 1998 arrived the fourth, and latest, incarnation of the now-long-running M16 line through the "M16A4". The iconic carrying handle was made removable and, in its place, a section of Picatinny rail could be fitted over the receiver - giving some customization to the weapon by allowing implementation of various optics, aimers and the like. Despite the changes, the general design lines of the M16 remained intact.
There have been many other derivatives witnessed throughout the M16 family line: the Colt Model 655 and Model 656 rifles were specialized sniper versions fielded during the Vietnam War and the XM177 was an early-form carbine model of the M16 also debuting during the conflict. The Model 733 continued the carbine approach and included features of the M16A2. The M231 "FPW" became a specialized "Firing Port Weapon" intended form armored vehicle occupants (primarily the M2 Bradley IFV). The Mark 12 was a DMR model while the C7 and C8 were Colt Canada offshoots for service with the Canadian Army. The Model 635 was similar in scope to the Colt Commando but chambered for the popular 9mm Parabellum pistol cartridge. The La France M16K was a modification of the M16 system intended for special forces elements and mated the M16 with a blowback system of operation.
More modern incarnations of the M16 include the M4 Carbine and the Heckler & Koch HK416 (both detailed elsewhere on this site). The M4 has largely succeeded the full-length M16A4 rifle in both Army and USMC service after combat experiences in Afghanistan and Iraq. The M4 is more compact and lighter while retaining much of the functionality of its larger cousin. It was introduced in 1994 and holds its own family of subvariants.
The HK 416, from German gun-maker Heckler & Koch, is marketed as an improvement on the M4 line while utilizing HK's in-house short-stroke gas piston developed for the full-length HK G36 assault rifle. The HK416 was introduced in 2004 and features several sections of rail for tactical accessories while retaining the 5.56x45mm NATO chambering. It also manages its own collection of subvariants and has already been featured in more modern American conflicts since its inception (it is said to be the weapon that killed Osama Bin Laden).
In one form or another, the M16 is set to remain a fixture on the modern battlefields for decades to come - rivaled only by the popularity and effectiveness of the AK-47 family which continues in service today and has spawned its own line of far-reaching weapon types - rifles, assault rifles, light machine guns, submachine guns, etc... Regardless, the M16 design holds its own level of popularity and is one of the more successful firearms of our time - production has now reached beyond 8 million units since 1959. Manufacture names include Colt's Manufacturing Company, Daewoo Precision Industries (South Korea), FN Herstal (Belgium), H&R Firearms, General Motors (Hydramat Division), Elisco, U.S. Ordnance and others.
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