Editing M16 rifle (section)

===M16A2===
{{Redirect|M16A2|the mine|M16 mine#Variants}}
[[File:Capt. John Arthur tackles the stress shoot event (7646423312).jpg|thumb|Spent case being deflected after firing an M16A2 (Model 705 with Safe/Semi/Burst trigger group) by a left-handed user.]]
[[File:M16A2 Enhanced with a complete trigger group.jpg|thumb|M16A2 Enhanced rifle (Model 708 with Safe/Semi/Burst/Auto trigger group)]]
[[File:M16a2m203 afmil.jpg|thumb|M16A2 with a heat shield hand guard and an [[M203 grenade launcher]] under it]]

The development of the M16A2 rifle was originally requested by the [[United States Marine Corps]] in 1979 as a result of combat experience in Vietnam with the M16A1.{{sfnp|Stevens|Ezell|1992|p=343}} It was officially adopted by the Department of Defense as the "Rifle, 5.56&nbsp;mm, M16A2" in 1983.{{sfnp|Rottman|2011|p=36}} The Marines were the first branch of the U.S. Armed Forces to adopt it, in the early/mid-1980s, with the [[United States Army]] following suit in 1986.{{sfnp|Rottman|2011|p=37}}

Modifications to the M16A2 were extensive. In addition to the then new STANAG 4172{{sfnp|NATO Standardization Office|2020|pp=100-125}} 5.56×45mm NATO chambering and its accompanying rifling, the barrel was made with a greater thickness in front of the front sight post, ostensibly to resist bending in the field and increase rigidity under sustained fire. The rest of the barrel was maintained at the original thickness for backwards compatibility with the M203 grenade launcher assembly. The barrel rifling was revised to a faster 1:7 (178&nbsp;mm) twist rate to adequately stabilize the new 5.56×45mm NATO SS109/M855 ball and L110/M856 tracer ammunition. The heavier longer SS109/M855 bullet reduced muzzle velocity from {{convert|3260|ft/s|m/s|0|abbr=on}}, to about {{convert|3110|ft/s|m/s|0|abbr=on}}{{sfnp|Miller|2004|pp=280-281}}

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 SS109/M855 rounds, and to allow windage adjustments without the need of a tool or cartridge.{{sfnp|Venola|2005|pp=6-18}} The flash suppressor was again modified, this time to be closed on the bottom, so the new birdcage-type muzzle device would not kick up dirt or snow when being fired from the prone position, and additionally act as an asymmetric recoil compensator to reduce [[muzzle climb]].{{sfnp|Hogg|Weeks|1985|p=196}}{{sfnp|''Retro Rifles''|2020}}

A spent case deflector was incorporated into the upper receiver immediately behind the ejection port to prevent (hot) cartridge cases from striking users shooting from the left shoulder.{{sfnp|Venola|2005|pp=6-18}} The action was also modified, replacing the original fully automatic setting with a three-round [[Burst mode (weapons)|burst]] setting.{{sfnp|Venola|2005|pp=6-18}} When using a fully automatic weapon, inexperienced 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.{{sfnp|Robinette|2017|p=1}} The number of rounds fired in a burst is determined by a [[Cam (mechanism)|cam]] mechanism that trips the trigger mechanism for each shot in the burst. For the burst the trigger must be held down for the full duration of the burst. The fire control group will terminate the burst if the trigger is released before the burst is complete but keep the cam in position. Thus, the next time the trigger is pulled, the weapon will only fire one or two rounds.{{sfnp|''AR15.com''|2004}}{{sfnp|YouTube, ''How 3-round Burst Works''|2014}} The US Army and USMC have both retired the M16A2, in favor of the newer M4A1 carbine and M16A4 respectively; a few M16A2s remain in service with the [[United States Army Reserve|U.S. Army Reserve]] and [[National Guard (United States)|National Guard]].{{sfnp|''National Guard Rifleman''|2018}}{{sfnp|''VOA News''|2020}}

The handguard was modified from the original triangular shape to a round one, which better fit smaller hands and could be retrofitted to older models of the M16. The new handguards were also symmetrical, so armories did not need to stock separate left- and right-hand spares. The handguard retention ring was also tapered to make it easier to install and uninstall the handguards.{{sfnp|Venola|2005|pp=6-18}}

The new buttstock became ten times stronger than the original due to advances in polymer technology since the early 1960s. Original M16 stocks were made from cellulose-impregnated phenolic resin; the newer M16A2 stocks were engineered from DuPont [[Zytel]] glass-filled thermoset polymers and became a replacement part for the preceding M16A1. The new buttstock was lengthened by {{convert|5/8|in|mm|sigfig=3|abbr=on}}, and whilst the hinged trapdoor and storage compartment inside the stock was retained, the anti-slip texturing now covers the entire buttplate for better grip on the shoulder.{{sfnp|Department of Defense|1991|p=144}}

The A2 barrel profile has a greater thickness from the front sight post to the muzzle, after bending in this area was reported in the field. Tests by the M16A2 team showed that this increased the resistance to intentional bending in this area by a factor 9. As to not further increase weight beyond acceptable levels, and to maintain commonality with the M203 grenade launcher, the rear half under the handguard maintained its original thin profile. After the new technical data was submitted, the team discovered that they had made an error in assessing the problem - the replaced bent barrels were not actually bent. 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. Cleaning this fouling caused the "bent" barrels to re-pass inspection. However this discovery came too late in the process to revert the change in profile.<ref>{{Cite web |last= |date=2018-09-03 |title=The Story of the Government Profile Barrel |url=https://www.everydaymarksman.co/equipment/government-profile-barrel/ |access-date=2024-07-17 |website=Everyday Marksman |language=en-US}}</ref>

A notch for the middle finger was added to the pistol grip as well as more aggressive texturing on the sides to enhance the grip, especially while wearing winter gloves. The new pistol grips were engineered from Zytel glass-filled thermoset polymers. The M16A2 pistol grip consequently became a replacement part for the preceding M16A1.{{sfnmp|1a1=Department of Defense|1y=1991|1p=144|2a1=Venola|2y=2005|2pp=6-18}}

While the standard M16A2 (Model 645 in Colt nomenclature) features a three position safe/semi/three-round burst selective fire control group, there also exists a four position safe/semi/three-round burst/automatic selective trigger pack for the Model 708 version of the rifle named "M16A2 Enhanced", used by some international customers like the [[32nd Marines Brigade (Greece)|32nd Marines Brigade]] of the Hellenic Army. It can be distinguished from the standard M16A2 by the presence of the embiggened fire selector lever, with a prominent triangular ridge and thumb dowel, as well as corresponding fire position markings on both sides of the receiver.{{sfnp|Hellenic Army|2002|pp=46-62}}<ref>{{Cite web |last=Ritchie |first=Cpl. Theodore W. |date=11 June 2009 |title=Defense.gov photo essay 090611-M-8752R-069.jpg |url=https://commons.wikimedia.org/wiki/File:Defense.gov_photo_essay_090611-M-8752R-069.jpg |access-date=17 March 2025 |website=commons.wikimedia.org}}</ref>