M16 rifle: Encyclopedia II - M16 rifle - Design

M16 rifle - Design

The M16's receivers are made of aluminum alloy, its barrel, bolt, and bolt carrier of steel, and its forestock, pistol grip, and buttstock of plastics. Early models were especially lightweight at 2.9 kg (6.4 lb). This was significantly less than older 7.62 mm "battle rifles" of the 1950s and 1960s. It also compares very favorably with the 5 kg (loaded) AK47. New models weigh more (8.5 lb or 3.9 kg) because of the "heavy" barrel profile in front of the front site. This was to prevent bending. The profile under the handguards remained the same for compatibility with clamp-on grenade launchers. The rifle is 40 inches (1.02 m) long with standard 20 inch (508 mm) barrel. Stoner experimented with titanium barrels in the prototype AR-10 for weight savings, but one barrel burst in the test. Stoner contended that the test had been rigged.

One distinctive ergonomic feature is a plastic or metal stock directly behind the action, which contains a recoil spring. This serves the dual function of operating spring and recoil buffer. The stock being in line with the bore reduces muzzle rise, especially during automatic fire. Because recoil does not significantly shift the point of aim, user fatigue is reduced.

Another distinctive ergonomic feature is that the main sight is on the top of a carry handle on top of the receiver. Newer models have a "flattop" upper receiver with a picatinny mounting rail to which the user can attach either a conventional carry handle/sighting system or numerous optical devices such as night vision scopes.

The M16 utilizes direct impingement gas operation; energy from high-pressure gas tapped from a port halfway down the barrel actuates the moving parts in the weapon. Combustion gases travel directly into a chamber in the bolt carrier behind the bolt itself, pushing the carrier away from the bolt. This reduces the number of moving parts by eliminating the need for a separate piston and cylinder.

The primary fault of direct impingement is that fouling and debris from expended gunpowder is blown directly into the breach. As the superheated combustion gas travels down the tube, it expands and cools, not unlike an aresol can cools when depressurized. This cooling causes vaporized matter to condense as it cools depositing a much greater volume of solids and doing so right on the operating parts in the action. Conversely, gas-piston operation uses high-pressure gasses for very short periods of time and keeps those gasses away from the breech. Because of this, the M16 requires more frequent cleaning and lubrication for reliable operation compared to gas piston operation.

The magazine release is on the right side of the rifle ahead of the trigger guard. Current military magazines have a capacity of 30 rounds, as opposed to the 20 round magazines issued in Vietnam. (30 round magazines were not developed and issued until late in the war.) Aftermarket double magazine clips are available. This practice is discouraged by military authorities because it is said to increase the chance that the top of a magazine will be damaged or pick up dirt. Nevertheless, recent pictures from Iraq (2004) show that Special Forces and mainstream military forces are quick to make use of double magazine clips.

The M193 bullet routinely fragments when it strikes soft tissue. When fired from less than 100 meters, the bullet will penetrate 100 mm (4 in) into body tissue, before yawing 90 to 180 degrees. Fragmentation occurs latteral forces on the bullet cause it to break in half. This occurs at the weak cannelure, which is a groove allowing the bullet casing to be sealed to the copper jacket. The rear section of the 5.56 mm bullet will then fragment into numerous tiny pieces, causing increased damage to surrounding tissue.

The U.S. does not, generally, issue fully-automatic M16-style rifles. Most M4 variants trade 3-round burst for automatic fire that is more effective in close-quarters weapon. Doctrinally, automatic fire is reserved for heavier machine guns that are designed to manage the recoil and deliver accurate fire even on fully-automatic. Fully-automatic fire would also expend ammunition faster, requiring soldiers to carry more to compensate. Also, the M16 barrel is too light to support sustained automatic fire and would overheat.

Most M16 and M4-style weapons issued are capable of semi-automatic fire. The burst-fire mechanism utilizes a three-part automatic sear that fires up to a three-round burst for each pull of the trigger. It is non resetting meaning that if a soldier fires a two-round burst and releases the trigger, he will encounter a single-round fired the next burst. Burst mechanisms provide a tradeoff between firepower, accuracy, and ammunition conservation. They do, however, limit the effectiveness of the M16 when used in suppression, ambush situations, and close-quarters combat.

Colt developed a variant with a heavier barrel, belt-feed, and fast barrel replacement. This was intended to provide a squad with the firepower of a machinegun with the portability of the M16. This was canceled in favor of the M249 Squad Automatic Weapon in the 1970s. Squads are now issued the M249 (one per fire team) to provide automatic fire. Fully-automatic versions (the M16A3 and M4A1) are issued by the U.S. military to certain specialized units.

Adapted from the Wikipedia article "Design", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki