[20] The M16A1 incorporated numerous modifications including a bolt-assist ("forward-assist"), chrome-plated bore, protective reinforcement around the magazine release, and revised flash hider.

[49] The 5.56 mm 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.

After testing the AR-15 with the ammunition manufactured by Remington that Armalite and Colt recommended, the Air Force declared that the AR-15 was its 'standard model' and ordered 8,500 rifles and 8.5 million rounds.

[27] Advocates for the AR-15 in the Defense Advanced Research Projects Agency acquired 1,000 Air Force AR-15s and shipped them to be tested by the Army of the Republic of Vietnam (ARVN).

[27] At this point, the bureaucratic battle lines were well-defined, with the Army ordnance agencies opposed to the AR-15 and the Air Force and civilian leadership of the Defense Department in favor.

However, General Maxwell D. Taylor, chairman of the Joint Chiefs of Staff, advised President John F. Kennedy that having two different calibers within the military system at the same time would be problematic and the request was rejected.

[66] United States Army Special Forces personnel filed battlefield reports lavishly praising the AR-15 and the stopping power of the 5.56 mm cartridge, and pressed for its adoption.

While the Olin WC 846 powder achieved the desired 3,300 ft (1,000 m) per second muzzle velocity, it produced much more fouling, which quickly jammed the M16's action (unless the rifle was cleaned well and often).

However, the rifle was initially delivered without adequate cleaning kits[44] or instructions because advertising from Colt asserted that the M16's materials made the weapon require little maintenance, leading to a misconception that it was capable of self-cleaning.

[44][77] Documented accounts of dead U.S. troops found next to disassembled rifles eventually led to a Congressional investigation:[78] We left with 72 men in our platoon and came back with 19.

Because the bolt is prevented from moving forward by the barrel, the carrier is driven to the rear by the expanding gases and thus converts the energy of the gas to the 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.

"[59] 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.

[95] Another underlying cause of the M16's jamming problem was identified by ordnance staff that discovered that Stoner and ammunition manufacturers had initially tested the AR-15 using DuPont IMR8208M extruded (stick) powder.

Requests included greater bullet lethality, newly built instead of rebuilt rifles, better-quality magazines, decreased weight, and a collapsible stock.

Later M16 models and the M16A1 had an improved rifling with six grooves, right-hand twist, one turn in 12 inches (1:304.8 mm or 54.8 calibers) for increased accuracy and was optimized to adequately stabilize the M193 ball and M196 tracer bullets.

M16A2 and current models are optimized for firing the heavier NATO SS109 ball and long L110 tracer bullets and have six grooves, right-hand twist, one turn in 7 in (1:177.8 mm or 32 calibers).

The unmarked approximately 0.070 in (1.78 mm) diameter aperture rear sight is for normal firing situations, zeroing and with the elevation knob for target distances up to 800 meters.

[128][note 6] "In Fallujah, Iraq 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.

[140] The 5.56×45mm NATO cartridge can also produce massive wounding effects when the bullet impacts at high speed and yaws ("tumbles") in tissue leading to fragmentation and rapid transfer of energy.

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

[182] Draft Standardization Agreement 4179 (STANAG 4179) was proposed to allow NATO members to easily share rifle ammunition and magazines down to the individual soldier level.

However, once established, the use of the system was expanded to other accessories, such as tactical lights, laser aiming modules, night vision devices, reflex sights, foregrips, bipods, and bayonets.

The new buttstock was lengthened by 5⁄8 in (15.9 mm) and included a fully textured polymer buttplate for better grip on the shoulder and retained a panel for accessing a small compartment inside the stock, often used for storing a basic cleaning kit.

When inspecting some using a borescope, they discovered that a burr left from drilling/reaming the gas port was causing copper fouling to build up at the front sight block, interfering with the gauge used.

Colt also produces M16A4 models for international purchases: A study of significant changes to Marine M16A4 rifles released in February 2015 outlined several new features that could be added from inexpensive and available components.

Those features included: a muzzle compensator in place of the flash suppressor to manage recoil and allow for faster follow-on shots, though at the cost of noise and flash signature and potential overpressure in close quarters; a heavier and/or free-floating barrel to increase accuracy from 4.5 MOA (Minute(s) Of Angle) to potentially 2 MOA; changing the reticle on the Rifle Combat Optic from chevron-shaped to a semi-circular reticle with a dot at the center used in the M27 IAR's Squad Day Optic so as not to obscure the target at long distance; using a trigger group with a more consistent pull force, even a reconsideration of the burst capability; and the addition of ambidextrous charging handles and bolt catch releases for easier use with left-handed shooters.

Officially adopted as a replacement for the M3 "Grease Gun" (and the Beretta M9 and M16A2 for select troops) in 1994, it was used with great success in the Balkans and in more recent conflicts, including the Afghanistan and Iraq theaters.

Other M5 variants and calibers are the: M5 SCW (Sub-compact weapon) (5.56×45mm); M5 300 (.300 AAC Blackout); M5 SMG (9×19mm); CMK (7.62×39mm); M7 Battle Rifle (7.62×51mm) and Designated Marksman and Semi-Automatic Sniper System (both 5.56×45mm).

Diemaco changed the trapdoor in the buttstock to make it easier to access and a spacer of 0.5 inches (13 mm) is available to adjust stock length to user preference, along with the addition of hammer-forged barrels.