Colt M16A2
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The development of the Colt M16A2 rifle was originally requested by the United States Marine Corps.[1] The Marines were the first branch of the US Armed Forces to adopt the M16A2. Modifications to the M16A2 were more extensive. In addition to the new rifling, the barrel was made with a greater thickness in front of the front sight post to resist bending in the field, allow for cooling between shots, and the stepped-up design allowed use of the M203 grenade launcher to be used with the weapon.[1] The front sight was now a square post with 4 detent positions in order to refine the sight picture. A new adjustable rear sight was added, allowing the rear sight to be dialed in for specific range settings between 300 and 800 meters to take full advantage of the ballistic characteristics of the new SS109 rounds and to allow windage adjustments without the need of a tool or cartridge.[1] The flash suppressor was again modified, this time to be closed on the bottom so it would not kick up dirt or snow when being fired from the prone position. The front grip was modified from the original triangular shape to a round one, which better fit smaller hands and could be fit to older models of M16s.[1] The new handguards were also symmetrical so that armories didn't need separate left and right spares. The handguard retention ring was angled to make it easier to install and uninstall the handguards.[1] The pistol grip adds a notch for the middle finger and more texture to enhance the grip. The buttstock was lengthened by 5/8 inches.[1] The new buttstock is ten times stronger than the original due to advances in polymer technology since the early 1960s. Original M16 stocks were made from fiberglass-impregnated resin; current stocks are engineered from DuPont Zytel glass-filled thermoset polymers. The new stock includes a fully textured polymer buttplate for better grip on the shoulder, and retains a panel for accessing a small compartment inside the stock often used for storing a basic cleaning kit. The heavier bullet reduces muzzle velocity from 3,200 feet per second (975 m/s), to about 3,050 feet per second (930 m/s). The A2 also uses a faster twist rifling to allow the use of a trajectory-matched tracer round. A spent case deflector was incorporated into the upper receiver to prevent casings from striking left-handed users.[1]
The action was also modified, replacing the fully-automatic setting with a three-round burst setting.[1] When using a fully-automatic weapon, poorly trained troops often hold down the trigger and "spray" when under fire. The U.S. Army concluded that three-shot groups provide an optimum combination of ammunition conservation, accuracy and firepower. There are mechanical flaws in the M16A2 burst mechanism. The trigger group does not reset when the trigger is released. If the user releases the trigger between the second and third round of the burst, for example, the next trigger pull would only result in a single shot. Even in semi-automatic mode, the trigger group mechanism affects weapon handling. With each round fired, the trigger group cycles through one of the three stages of the burst mechanism. Worse, the trigger pull at each of these stages may vary as much as 6 lbs. in pressure differential, detracting from accuracy.
All together, the M16A2s new features added weight and complexity to the M16 series. Critics also point out that neither of the rear sight apertures is ideally sized. The smaller aperture was described as being too small, making quick acquisition of the front sight post difficult; and the larger aperture was described as being too large, resulting in decreased accuracy. To make matters worse, the rear sight apertures are not machined to be on the same plane. In other words, the point of impact changes when the user changes from one aperture to the other. The rear sight's range adjustment feature is rarely used in combat as soldiers tend to leave the rear sight on its lowest range setting: 300 meters. Despite criticism, a new rifle was needed both to comply with NATO standardization of the SS109 (M855) and to replace aging Vietnam era weapons in the inventory.
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