[39][41][42][43][44][40] This new cartridge was developed by shortening the standard 7.92×57mm Mauser round and giving it a lighter 125-grain bullet that limited range but allowed for more controllable automatic fire.

The principle of this weapon — the reduction of muzzle impulse to get usable automatic fire within the actual ranges of combat — was probably the most important advance in small arms since the invention of smokeless powder.

"[34] On the other hand, the U.S. Army was influenced by combat experience with semi-automatic weapons such as the M1 Garand and M1 Carbine, which enjoyed a significant advantage over enemies armed primarily with bolt-action rifles.

Springfield Armory's T44E4 and heavier T44E5 were essentially updated versions of the Garand chambered for the new 7.62 mm round, while Fabrique Nationale submitted their FN FAL as the T48.

ArmaLite entered the competition late, hurriedly submitting several AR-10 prototype rifles in the fall of 1956 to the United States Army's Springfield Armory for testing.

[76][77] In the end, the United States Army chose the T44, now called the M14 rifle[70]–an improved M1 Garand with a 20-round magazine and automatic fire capability.

[44] The 5.56mm round had to penetrate a standard U.S. helmet at 500 yards (460 meters) and retain a velocity over the speed of sound while matching or exceeding the wounding ability of the .30 Carbine cartridge.

"[36][88][89] It also led to the development of small-caliber high-velocity service rifles by every major army in the world, including the USSR and the People's Republic of China.

[36] It was designed to be manufactured with the extensive use of aluminum and synthetic materials by state-of-the-art Computer Numerical Control (CNC) automated machinery.

It was designed to be a simple, reliable automatic rifle that could be manufactured quickly and cheaply, using mass production methods that were state of the art in the Soviet Union during the late 1940s.

[117] Because of its stamped-steel design, it is not possible to manufacture the AK-47 series efficiently in small plants, due to the large amount of metal stamping equipment needed for mass production.

The receivers, produced from 7075 T6 aircraft aluminum, which helps keep the rifle lightweight and dissipates heat better than conventional metals, are hard-anodized with a non-reflective matte gray weather-resistant finish.

This enhanced performance was due to several factors, not the least of which is the ability to simply insert the magazine into the AR in a conventional manner rather than the "rock and lock" method required with the AK platform.

[132] The M16 has a well-designed safety lever located on the left side of the weapon that is easily manipulated by the user's thumb while maintaining a strong hold on the pistol grip.

The primary disadvantage to this system is the disruption of the point of aim due to the center of mass changing during the action cycle and energetic and abrupt stops at the beginning and end of bolt carrier travel.

[137] The current United States Army and Air Force issue M4 Carbine comes with the M68 Close Combat Optic and Back-up Iron Sight.

[174][130] "In Fallujah, Marines with ACOG-equipped M16A4s created a stir by taking so many headshots that until the wounds were closely examined, some observers thought the insurgents had been executed.

[174][177][178] "At 300 meters, expert shooters (firing AK-47s) at prone or at bench rest positions had difficulty putting ten consecutive rounds on target.

[58] In June 2010, the United States Army announced it began shipping its new 5.56mm, lead-free, M855A1 Enhanced Performance Round to active combat zones.

[197] This upgrade is designed to maximize the performance of the 5.56×45mm round, extend range, improve accuracy, increase penetration, and consistently fragment in soft-tissue when fired from not only standard-length M16s but also the short-barreled M4 carbines.

[146]Note **: Both the AK-47 and the M16 will overheat fairly quickly under normal combat conditions and have a sustained rate of fire as low as 12 to 15 rounds per minute (about the same as a bolt-action rifle).

Because the bolt is prevented from moving forward by the barrel, the carrier is driven to the rear by the expanding gasses and thus converts the energy of the gas to movement of the rifle’s parts.

This accumulating carbon and vaporized metal build-up within the receiver and bolt carrier negatively affects reliability and necessitates more intensive maintenance on the part of the individual soldier.

The internal piston operation increases the amount of heat that is deposited in the receiver while firing the M16 and causes the essential lubricant to be "burned off".

)"[87] In March 1970, the "President’s Blue Ribbon Defense Panel" concluded that the issuance of the M16 saved the lives of 20,000 U.S. servicemen during the Vietnam War, who would have otherwise died had the M14 remained in service.

[277] "Unlike previous magazines, the EPM uses a new, modified magazine body (and therefore not interchangeable, hence the different color) which presents the rounds at a more favorable angle to the rifle’s feed way, improving reliability and, importantly, preventing the hardened steel tips of new 5.56mm M855A1 Enhanced Performance Rounds from contacting the aluminum feed ramp section on M4 type rifles.

[48] The AK-47 was designed to be a cheap, simple, easy-to-manufacture assault rifle,[222] perfectly matching Soviet military doctrine that treats equipment and weapons as disposable items.

[287][288][289] The M16 was designed to be a serviceable assault rifle,[290] perfectly matching American military doctrine where units are resupplied continuously and are expected to perform most of their maintenance and repairs in the field.

"[16][340] Nevertheless, during the Afghan war, the Mujahedeen called the 5.45×39mm round the "Poison Bullet" due to the severe wounds it produced to extremities and the resulting need to amputate.

[8] Additional information can be found in "Rifle Evaluation Study", United States Army, Infantry Combat Developments Agency, ADA050268, 8 Dec 1962".