Editing M16 rifle (section)

==Variants==
{{Further|topic=M16 variants|List of Colt AR-15 and M16 rifle variants}}

===M16===
[[File:USIA 64-116.jpg|thumb|An early M16 rifle without forward-assist. Note: "duckbill" flash suppressor and triangular handguard.]]

This was the first M16 variant adopted operationally, originally by the U.S. Air Force. It was equipped with triangular handguards, buttstocks without a compartment for the storage of a cleaning kit,{{sfnp|Ezell|Pegg|Smith|Smith|1993|pp=746–762}} a three-pronged "duckbill" flash suppressor designed to preserve the shooter's night vision by disrupting the flash, full auto, and no forward assist. The M16 has a safe/semi/auto selective fire trigger group. Bolt carriers were originally chrome plated and slick-sided, lacking forward assist notches. Later, the chrome-plated carriers were dropped in favor of Army-issued notched and [[parkerized]] carriers, though the interior portion of the bolt carrier is still chrome-lined. The barrel rifling had a 1:12 (305&nbsp;mm) [[Rifling#Twist rate|twist rate]] to adequately stabilize the M193 ball and M196 tracer ammunition. The Air Force continued to operate these weapons until around 2001, at which time the Air Force converted all of its M16s to the M16A2 configuration.

The M16 was also adopted by the [[Special Air Service|British SAS]], who used it during the [[Falklands War]].{{sfnp|Special Operations.Com ''SAS Weapons''|2000}}

===XM16E1 and M16A1 (Colt Model 603)===
{{Redirect|M16A1|the mine|M16 mine#Variants}}
[[File:NARA 111-CCV-349-CC43120 101st Airborne soldiers firing XM148 Operation Cook 1967.jpg|thumb|XM16E1 with [[XM148 grenade launcher]] ]]
[[File:M16A1 brimob.jpg|thumb|M16A1 rifle with 30-round magazine]]

The U.S. Army XM16E1 was essentially the same weapon as the M16 with the addition of a forward assist and corresponding notches in the chrome bolt carrier. A rib was built into the side of the receiver to help prevent accidentally pressing the magazine release button while closing the ejection port cover.

The M16A1 was the finalized production model and was produced from February 1967 until 1982. To address issues with the XM16E1, for the M16A1 a closed, birdcage symmetric flash suppressor with open side slots to the top, bottom, left and right replaced the XM16E1's three-pronged flash suppressor, which caught on twigs and leaves, from 1967 onwards.{{Sfnp|''Retro Rifles''|2020}} Various other changes were made after numerous problems in the field. Cleaning kits were developed and issued, while barrels with chrome-plated chambers and later fully lined bores were introduced. A small storage compartment inside the stock was introduced.{{Sfnp|Meehan|1985|pp=145-147}} Covered by a trapdoor with checkered anti-slip texturing, it is often used for storing a basic cleaning kit. To promote reliability and durability, the mechanical behavior of the operating system was revised to make it compatible for using US military issued ammunition loaded with WC846 ball powder (which reaches peak pressure significantly quicker than the extruded IMR8208M powder and increases the cyclic rate of fire for which the operating system was originally designed). Revisions like reducing the diameter of the gas port to mitigate the higher port pressure caused by the ball powder to properly gas the operating system again, updating the buffer assembly, changing the bolt carrier surface finish to manganese phosphate and the gas tube material to stainless steel contributed to improved mechanical behavior.

The rib was extended on production M16A1s to help in preventing the magazine release from inadvertently being pressed. The hole in the bolt that accepts the cam pin was crimped inward on one side, in such a way that the cam pin may not be inserted with the bolt installed backwards, which would cause failures to eject until corrected. With these and other changes, the malfunction rate slowly declined, and new soldiers were generally unfamiliar with early problems.

The M16A1 saw limited use in training capacities until the early 2000s,{{Sfnp|''DOD News, 1 March 2010''}}{{Sfnp|''DOD News, 1 July 2000''}}{{Sfnp|''Navy News, 29 December 2017''}} but is no longer in active service with the U.S., although is still standard issue in many world armies.

===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>

===M16A3===
[[File:US Navy 071130-N-0555B-073 Aviation Boatswain's Mate (handling) Airman Jaime Chamorro fires an M-16.jpg|thumb|M16A3 with a Safe/Semi/Auto trigger group]]

The M16A3 is a modified version of the M16A2 adopted in small numbers by the U.S. Navy [[United States Navy SEALs|SEALs]], [[Seabee (United States Navy)|Seabees]], some US security units,{{sfnp|''Defense Industry Daily''|2008}} and the [[Philippine National Police]].<ref name="auto">{{cite web | url=https://pmddtc.state.gov/reports/congnotices/108/CN006-04.pdf | archive-url=https://web.archive.org/web/20111016225158/https://pmddtc.state.gov/reports/congnotices/108/CN006-04.pdf | archive-date=16 October 2011 | title=Home - DDTC Public Portal - DDTC Public Portal }}</ref> It features the M16A1 selective fire control group providing "safe", "semi-automatic" and "fully automatic" modes, and type-specific markings on the magazine well. Otherwise, it is externally identical to the M16A2, and should not be conflated with rifles built by armorers using existing full-auto lower receiver assemblies, or M16A1s retrofitted with newer M16A2 furniture.

===M16A4===
[[File:M16A4-JH01 noBG.jpg|thumb|right|M16A4 rifle with a removable carrying handle, polymer handguards and [[M7 bayonet]] mounted]]
[[File:The M16A4 Series 5.56mm Rifle.jpg|thumb|right|M16A4 rifle with ACOG sight, railed hand guard and foregrip]]

Adopted in July 1997, the M16A4 is the fourth generation of the M16 series. It is equipped with a removable carrying handle and [[Picatinny rail]] for mounting optics and other ancillary devices.{{sfnp|Green|Stewart|2004|pp=16-17}} The M16A4 rear aperture sights are adjustable from {{convert|300|m|yd|abbr=on}} up to {{convert|600|m|yd|abbr=on}}, where the further similar M16A2 iron sights line can reach up to {{convert|800|m|yd|abbr=on}}. The introduction of the Picatinny rail required the use of a higher F-marked front sight base to raise the post. The FN M16A4, with safe/semi/three-round burst selective fire modes, became standard issue for the U.S. Marine Corps.

Colt also produces M16A4 models for international purchases:
* R0901 / RO901/ [[National Stock Number|NSN 1005-01-383-2872]] (Safe/Semi/Auto)
* R0905 / RO905 (Safe/Semi/Burst)

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.{{sfnp|''Marine Corps Times'', February 16, 2015, p.4}}

In 2014, Marine units were provided with a limited number of adjustable stocks in place of the traditional fixed stock for their M16A4s to issue to smaller Marines who would have trouble comfortably reaching the trigger when wearing body armor. The adjustable stocks were added as a standard authorized accessory, meaning units can use operations and maintenance funds to purchase more if needed.{{sfnp|''Marine Corps Times'', 17 September 2015, p.7}}

The Marine Corps had long maintained the full-length M16 as their standard infantry rifle, but in October 2015 the switch to the M4 carbine was approved as the standard-issue weapon, giving Marine infantry a smaller and more compact weapon. Enough M4s were already in the inventory to re-equip all necessary units by September 2016, and all M16A4s were moved to support{{sfnp|''Marine Corps Times'', 26 October 2015, p.19}}{{sfnp|''Military Times'', 27 July 2015, p.3}} and non-infantry Marines.{{sfnp|''Marine Corps Times'', 1 November 2015, p.5}}

===Summary of differences===
{| class="wikitable"
|-
! Colt model no.
! Military designation
! 20-in barrel w/ bayonet lug
! Handguard type
! Buttstock type
! Pistol grip type
! Lower receiver type
! Upper receiver type
! Rear sight type
! Front sight type
! Muzzle device
! Forward assist?
! Case deflector?
! Trigger pack
|-
|601
|AR-15 designated M16 on 3 December 1963
|A1 profile (1:14 twist)
|Green or brown full-length triangular
|Green or brown fixed A1
|A1
|A1
|A1
|A1
|A1
|Duckbill flash suppressor
|No
|No
|Safe/Semi/Auto
|-
|602
|AR-15 designated M16 3 December 1963 – there was no XM16
|A1 profile (1:12 twist)
|Full-length triangular
|Fixed A1
|A1
|A1
|A1
|A1
|A1
|Duckbill or three-prong flash suppressor
|No
|No
|Safe/Semi/Auto
|-
|603
|XM16E1 designation changed to M16A1 in February 1967 with standardization
|A1 profile (1:12 twist)
|Full-length triangular
|Fixed A1
|A1
|A1
|A1
|A1
|A1
|Three-prong or M16A1 birdcage flash suppressor
|Yes
|No
|Safe/Semi/Auto
|-
|603
|M16A1
|A1 profile (1:12 twist)
|Full-length triangular
|Fixed A1
|A1
|A1
|A1
|A1
|A1
|Three-prong or birdcage flash suppressor
|Yes
|Yes or No
|Safe/Semi/Auto
|-
|604
|M16
|A1 profile (1:12 twist)
|Full-length triangular
|Fixed A1
|A1
|A1
|A1
|A1
|A1
|Three-prong or M16A1-style birdcage flash suppressor
|No
|No
|Safe/Semi/Auto
|-
|645
|M16A1E1/PIP
|A2 profile (1:7 twist)
|Full-length ribbed
|Fixed A2
|A1
|A1 or A2
|A1 or A2
|A1 or A2
|A2
|M16A1 or M16A2-style birdcage flash suppressor
|Yes
|Yes or no
|Safe/Semi/Auto or Safe/Semi/Burst
|-
|645
|M16A2
|A2 profile (1:7 twist)
|Full-length ribbed
|Fixed A2
|A2
|A2
|A2
|A2
|A2
|M16A2-style birdcage flash suppressor
|Yes
|Yes
|Safe/Semi/Burst or Safe/Semi/Burst/Auto
|-
|708
|M16A2 Enhanced
|A2 profile (1:7 twist)
|Full-length ribbed
|Fixed A2
|A2
|A2
|A2
|A2
|A2
|M16A2-style birdcage flash suppressor
|Yes
|Yes
|Safe/Semi/Burst/Auto
|-
|645E
|M16A2E1
|A2 profile (1:7 twist)
|Full-length ribbed
|Fixed A2
|A2
|A2
|Flattop with Colt rail
|Flip-up
|Folding
|M16A2-style birdcage flash suppressor
|Yes
|Yes
|Safe/Semi/Burst or Safe/Semi/Burst/Auto
|-
|N/A
|M16A2E2
|A2 profile (1:7 twist)
|Full-length semi-beavertail w/ HEL guide
|Retractable ACR
|ACR
|A2
|Flattop with Colt rail
|None
|A2
|ACR muzzle brake
|Yes
|Yes
|Safe/Semi/Burst or Safe/Semi/Burst/Auto
|-
|646
|M16A3 (M16A2E3)
|A2 profile (1:7 twist)
|Full-length ribbed
|Fixed A2
|A2
|A2
|A2
|A2
|A2
|M16A2-style birdcage flash suppressor
|Yes
|Yes
|Safe/Semi/Auto
|-
|655
|M16A1 Special High Profile
|HBAR profile <br />(1:12 twist)
|Full-length triangular
|Fixed A1
|A1
|A1
|A1
|A1
|A1
|M16A1-style birdcage flash suppressor
|Yes
|No
|Safe/Semi/Auto
|-
|656
|M16A1 Special Low Profile
|HBAR profile <br />(1:12 twist)
|Full-length triangular
|Fixed A1
|A1
|A1
|A1 with modified Weaver base
|Low Profile A1
|Hooded A1
|M16A1-style birdcage flash suppressor
|Yes
|No
|Safe/Semi/Auto
|-
|945
|M16A4 (M16A2E4)
|A2 profile (1:7 twist)
|Full-length ribbed or KAC M5 RAS
|Fixed A2/M4 collapsed stock
|A2
|A2
|Flattop with MIL-STD-1913 rail
|None
|A4
|M16A2-style birdcage flash suppressor
|Yes
|Yes
|Safe/Semi/Auto (RO901) or Safe/Semi/Burst (RO905)
|}