Editing M16 rifle (section)

==Design==
[[File:Rifle 5.56mm, XM16E1, Operation and Cycle of Functioning TF9-3663.webm|thumb|Video of Rifle 5.56mm, XM16E1, Operation and Cycle of Functioning]]
[[File:M16 rifle Firing FM 23-9 Fig 2-7.png|thumb|M16 internal piston action system]]

The M16 is a lightweight, 5.56&nbsp;mm, air-cooled, [[Gas-operated reloading|gas-operated]], [[Magazine (firearms)|magazine]]-fed [[assault rifle]], with a [[rotating bolt]]. The M16's receivers are made of [[7075 aluminium alloy|7075 aluminum alloy]], its barrel, bolt, and bolt carrier of steel, and its handguards, pistol grip, and buttstock of plastics.

The M16 internal piston action was derived from the original ArmaLite AR-10 and ArmaLite AR-15 actions. This internal piston action system designed by Eugene Stoner is commonly called a direct impingement system, but it does not use a conventional direct impingement system. In {{US Patent|2951424}}, the designer states: ″This invention is a true expanding gas system instead of the conventional impinging gas system.″{{sfnp|''Patent US2951424''|1956}} The gas system, bolt carrier, and bolt-locking design is ammunition specific, since it does not have an adjustable gas port or valve to adjust the weapon to various propellant and projectile or barrel length specific pressure behavior.

The M16A1 was especially lightweight at {{convert|7.9|lb|kg}} with a loaded 30-round magazine.{{sfnp|Meehan|1985|p=32}} This was significantly less than the M14 that it replaced at {{convert|10.7|lb|kg}} with a loaded 20-round magazine.{{sfnp|Lambert|1965|p=4}} It is also lighter when compared to the [[AKM]]'s {{convert|8.3|lb|kg}} with a loaded 30-round magazine.{{sfnp|''AK-47 Technical Description''|2010}}

The M16A2 weighs {{convert|8.8|lb|kg|1|abbr=on}} loaded with a 30-round magazine, because of the adoption of a thicker barrel profile. The thicker barrel is more resistant to damage when handled roughly and is also slower to overheat during sustained fire. Unlike a traditional "bull" barrel that is thick its entire length, the M16A2's barrel is only thick forward of the handguards. The barrel profile under the handguards remained the same as the M16A1 for compatibility with the [[M203 grenade launcher]].

===Barrel===
Early model M16 barrels had a [[rifling]] twist of four grooves, right-hand [[twist rate|twist]], one turn in 14&nbsp;inches (1:355.6&nbsp;mm or 64 calibers) bore—as it was the same rifling as used by the [[.222 Remington]] sporting cartridge. After finding out that under unfavorable conditions, military bullets could yaw in flight at long ranges, the rifling was soon altered. Later M16 models and the M16A1 had an improved rifling with six grooves, right-hand twist, one turn in 12&nbsp;inches (1:304.8&nbsp;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&nbsp;mm or 32 calibers).{{sfnp|Colt, ''M16 5.56mm Rifle''|2003}}{{sfnmp|1a1=Cooke|1y=2005|1p=1|2a1=Hickerson|2y=1991|2p=9|3a1=Hogg|3a2=Weeks|3y=1985|3pp=195-196|4a1=Simpson|4y=2011|4p=1}}

M193 ball and M196 tracer bullets may be fired in a rifle with a one turn in 7 in (1:177.8&nbsp;mm or 32 calibers) twist barrel. NATO SS109 ball and L110 tracer bullets should only be used in emergency situations at ranges under {{convert|90|m|yd|0|abbr=on}} with a one turn in 12&nbsp;inches (1:304.8&nbsp;mm or 54.8 calibers) twist, as this twist is insufficient to stabilize these projectiles.{{sfnp|Cooke|2005|p=1}}
Weapons designed to adequately stabilize both the M193 or SS109 projectiles (like civilian market clones) usually have a six-groove, right-hand twist, one turn in 9&nbsp;inches (1:228.6&nbsp;mm or 41.1 calibers) or one turn in 8&nbsp;inches (1:203.2&nbsp;mm or 36.5 calibers) bore, although other and 1:7&nbsp;inches twist rates are available as well.

===Recoil===
{{blockquote|The (M16's) Stoner system provides a very symmetric design that allows straight-line movement of the operating components. This allows recoil forces to drive straight to the rear. Instead of connecting or other mechanical parts driving the system, high-pressure gas performs this function, reducing the weight of moving parts and the rifle as a whole.{{sfnp|''Armalite''|2010}} }}
The M16 uses a "straight-line" recoil design, where the recoil spring is located in the stock directly behind the action,{{sfnp|Ezell|Pegg|Smith|Smith|1993|pp=746–762}} and serves the dual function of operating spring and recoil buffer.{{sfnp|Ezell|Pegg|Smith|Smith|1993|pp=746–762}} The stock being in line with the bore also reduces muzzle rise, especially during automatic fire. Because recoil does not significantly shift the point of aim, faster follow-up shots are possible and user fatigue is reduced. In addition, current model M16 flash-suppressors also act as compensators to reduce recoil further.{{sfnp|Meehan|1985|p=32}}

{| class="wikitable" style="margin: 1em auto; text-align:center; width:350px;"
|-
! colspan=2 |Free recoil{{sfnp|''KS, Recoil Calculator''}}
|-
!
! M16
|-
|'''Momentum'''
| style=|40.4&nbsp;lb-ft/s
|-
|'''Velocity'''
| style=|{{convert|5.1|ft/s|m/s|abbr=on}}
|-
|'''Energy'''
| style=|{{convert|3.2|ftlb|J|abbr=on|adj=off}}
|}
Notes: Free recoil is calculated by using the rifle weight, bullet weight, muzzle velocity, and charge weight.{{sfnp|''KS, Recoil Calculator''}} It is that which would be measured if the rifle were fired suspended from strings, free to recoil.{{sfnp|''KS, Recoil Calculator''}} A rifle's perceived recoil is also dependent on many other factors which are not readily quantified.{{sfnp|''KS, Recoil Calculator''}}

===Sights===
[[File:M16 rifle correct sight picture fig 4-18.png|thumb|M16 sight picture when using the {{convert|0.070|in|mm|2|abbr=on}} rear aperture]]
[[File:M16A2 rear sight in front side P1010034.JPG|thumb|M16A2 with unmarked aperture rear sight for normal firing situations raised. The larger aperture, marked '0-2', is flipped down.]]
[[File:M16 upper right.jpg|thumb|Fully adjustable rear sight, brass deflector and forward assist of the M16A2]]

The M16's most distinctive ergonomic feature is the carrying handle and rear sight assembly on top of the receiver. This is a by-product of the original AR-10 design, where the carrying handle contained a rear sight that could be set for specific range settings and also served to protect the charging handle.{{sfnp|Ezell|Pegg|Smith|Smith|1993|pp=746–762}}
The M16 carry handle also provided mounting groove interfaces and a hole at the bottom of the handle groove for mounting a Colt 3×20 [[telescopic sight]] featuring a [[Telescopic sight#Bullet drop compensation|Bullet Drop Compensation]] elevation adjustment knob for ranges from {{convert|100|to|500|yd|m|0|abbr=on}}. This concurs with the pre-M16A2 maximum effective range of {{convert|460|m|yd|0|abbr=on}}.{{sfnp|Hickerson|1991|p=9}} The Colt 3×20 telescopic sight was factory adjusted to be parallax-free at {{convert|200|yd|m|0|abbr=on}}.{{sfnp|RBR, ''Early Colt 3× Scope''|2020}}{{sfnp|RBR, ''Late Colt 3× Scope''|2020}} In Delft, the Netherlands [[Artillerie-Inrichtingen]] produced a roughly similar 3×25 telescopic sight for the carrying handle mounting interfaces.{{sfnp|RBR, ''Delft 3x25 Scope''|2020}}

The M16 elevated iron sight line has a {{convert|19.75|in|mm|0|abbr=on}} sight radius.{{sfnp|Colt, ''M16 5.56mm Rifle''|2003}} As the M16 series rear sight, front sight and sighting in targets designs were modified over time and non-iron sight (optical) aiming devices and new service ammunition were introduced zeroing procedures changed.{{sfnp|ARM, ''New Zeroing Procedures''|2018}}

The standard pre-M16A2 "Daylight Sight System" uses an [[ArmaLite AR-15#Design details|AR-15-style]] L-type flip, two [[Iron sights#Aperture sights|aperture rear sight]] featuring two combat settings: short-range {{convert|0|to|300|m|yd|0|abbr=on}} and long-range {{convert|300|to|400|m|yd|0|abbr=on}}, marked 'L'.{{sfnp|Meehan|1985|pp=50-52}} The pre-M16A2 "Daylight Sight System" short-range and long-range zeros are {{convert|250|and|375|m|yd|0|abbr=on}} with M193 ammunition.{{sfnp|Meehan|1985|pp=50-52}} The rear sight features a windage drum that can be adjusted during zeroing with about 1 MOA increments. The front sight is a tapered round post of approximately {{convert|0.0625|in|mm|2|abbr=on}} diameter adjustable during zeroing in about 1 [[Minute and second of arc#Firearms|MOA]] increments. A cartridge or tool is required to (re)zero the sight line.{{sfnmp|1a1=Fitchett|1y=2023|1p=1|2a1=Meehan|2y=1985|2pp=50-52}}{{sfnp|ARM, ''New Zeroing Procedures''|2018}}

An alternative pre-M16A2 "Low Light Level Sight System", includes a front sight post with a weak light source provided by [[tritium radioluminescence]] in an embedded small glass vial and a two aperture rear sight consisting of a {{convert|2|mm|in|3|abbr=on}} diameter aperture marked 'L' intended for normal use out to {{convert|460|m|yd|0|abbr=on}} and a {{convert|7|mm|in|3|abbr=on}} diameter large aperture for night firing.{{sfnp|Meehan|1985|pp=50-52}} Regulation stipulates the radioluminescent front sight post must be replaced if more than 144 months (12 years) elapsed after manufacture.{{sfnp|Hickerson|1991|p=145}} The "Low Light Level Sight System" elevation and windage adjustment increments are somewhat coarser compared to the "Daylight Sight System".{{sfnp|Meehan|1985|pp=50-52}}

With the advent of the M16A2, a less simple fully adjustable rear sight was added, allowing the rear sight to be dialed in with an elevation wheel for specific range settings between {{convert|300|and|800|m|yd|0|abbr=on}} in 100 m increments and to allow windage adjustments with a windage knob without the need of a cartridge or tool. The unmarked approximately {{convert|0.070|in|mm|2|abbr=on}} diameter aperture rear sight is for normal firing situations, zeroing and with the elevation knob for target distances up to 800 meters. The downsides of relatively small rear sight apertures are less light transmission through the aperture and a reduced field of view. A new larger approximately {{convert|0.2|in|mm|2|abbr=on}} diameter aperture, marked '0-2' and featuring a windage setting index mark, offers a larger field of view during battle conditions and is used as a [[ghost ring]] for quick target engagement and during limited visibility. When flipped down, the engraved windage mark on top of the '0-2' aperture ring shows the dialed in windage setting on a windage scale at the rear of the rear sight assembly. When the normal use rear aperture sight is zeroed at 300 m with SS109/M855 ammunition, first used in the M16A2, the '0-2' rear sight will be zeroed for 200 m. The front sight post was widened to approximately {{convert|0.075|in|mm|2|abbr=on}} diameter and became square and became adjustable during zeroing in about 1.2 MOA increments.{{sfnmp|1a1=Meehan|1y=1985|1p=53|2a1=Venola|2y=2005|2pp=6-18}}{{sfnp|''The New Rifleman''| 2016}}

The M16A4 omitted the carrying handle and rear sight assembly on top of the receiver. Instead, it features a MIL-STD-1913 [[Picatinny rail]]ed flat-top upper receiver for mounting various optical sighting devices or a new detachable carrying handle and M16A2-style rear sight assembly. The current U.S. Army and Air Force issue M4A1 carbine comes with the [[Aimpoint CompM2|M68 close combat optic]] and back-up [[iron sight]].{{sfnmp|1a1=Department of the Air Force|1y=2004|1p=183|2a1=Henderson|2y=2010|2p=1}} The U.S. Marine Corps uses the [[Advanced Combat Optical Gunsight|ACOG]] Rifle Combat Optic{{sfnp|''Defense Industry Daily''|2005}}{{sfnp|''Trijicon®''|2011}} and the U.S. Navy uses the [[EOTech]] [[holographic weapon sight]].{{sfnp|Hopkins|2009|p=1}}

===Range and accuracy===
The M16 rifle is considered to be very accurate for a service rifle.{{sfnmp|1a1=Dyer|1y=2014|1p=122|2a1=Halberstadt|2y=2008|2p=211–212|3a1=Newick|3y=1989|3pp=26–27|4a1=Rottman|4y=2011|4p=38}} Its light recoil, high-velocity and flat trajectory allow shooters to take headshots out to 300 meters.{{sfnmp|1a1=Castaneda|1y=2006|1p=2|2a1=Sweeney|2y=2007|2p=5}} Newer M16s use the newer M855 cartridge increasing their effective range to 600 meters.{{sfnp|Colt, ''M16 5.56mm Rifle''|2003}} They are more accurate than their predecessors and are capable of shooting 1–3-inch groups at 100 yards.{{sfnp|ADC, ''Avtomat Kalashnikov''|1998}}{{NoteTag|Per Taylor "From an accuracy standpoint, there is no comparison between the M16 and AK. As long as the upper and lower receivers are tight and the trigger is halfway decent, the M16 is capable of MOA accuracy, whereas a typical AK will produce 5 to 6 MOA at best. And, if a free-floated barrel is incorporated to the M16's design, it becomes capable of {{frac|1|2}}-MOA or better, making it fully as accurate as a finely tuned heavy-barreled bolt-action precision rifle."{{sfnp|Taylor|2009|p=1}} }} "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."{{sfnp|Venola|2004|pp=22–30}} The newest M855A1 EPR cartridge is even more accurate and during testing "...has shown that, on average, 95 percent of the rounds will hit within an 8 × 8-inch (20.3 × 20.3 cm) target at 600 meters."{{sfnp|Woods|2010|p=135}}

{| class="wikitable" style="margin: 1em auto; text-align:center;"
|- valign="top"
! rowspan="2" |Rifle
! rowspan="2" |Caliber
! rowspan="2" |Cartridge
! rowspan="2" |Cartridge <br />weight
! rowspan="2" |Bullet <br />weight
! rowspan="2" |Velocity
! rowspan="2" |Energy
! colspan="4" |Range
! colspan="2" |Accuracy
|-
! <small>Effective{{NoteTag|The effective range of a firearm is the maximum distance at which a weapon may be expected to be accurate and achieve the desired effect.{{sfnp|Joint Staff, J-7|2005|p=269}} }}</small>
! <small>Horizontal{{NoteTag|The horizontal range is the distance traveled by a bullet, fired from the rifle at a height of 1.6 meters and 0° elevation, until the bullet hits the ground.{{sfnp|Ingalls|1893|p=7}} }}</small>
! <small>Lethal{{NoteTag|The lethal range is the maximum range of a small-arms projectile, while still maintaining the minimum energy required to put unprotected personnel out of action, which is generally believed to be 15 kilogram-meters (147&nbsp;J / 108&nbsp;ft.lbf).{{sfnp|Kjellgren|1970|pp=40–44}} This is the equivalent of the muzzle energy of a .22LR handgun.{{sfnp|Winchester, ''X22LRSS1''|2012}} }}</small>
! <small>Maximum{{NoteTag|The maximum range of a small-arms projectile is attained at about 30° elevation. This maximum range is only of safety interest, not for combat firing.}}{{sfnp|Kjellgren|1970|pp=40–44}}</small>
! <small>10 shot group <br />@ 100 meters</small>
! <small>10 shot group <br />@ 300 meters</small>
|-
!M16
|[[5.56×45mm|5.56×45&nbsp;mm]]
|M193
|184 gr <br />(11.9 g){{sfnp|Kjellgren|1970|pp=40–44}}
|55 gr <br />(3.6 g){{sfnp|Hartink|2004|p=121-123}}
|3,250&nbsp;fps <br />(990&nbsp;m/s){{sfnp|Hartink|2004|p=121-123}}
|1,302&nbsp;ft/lb <br />(1,764 J){{sfnp|Hartink|2004|p=121-123}}
|500 yds <br />(460&nbsp;m){{sfnp|Meehan|1985|p=50}}
|711 yds <br />(650&nbsp;m){{sfnp|Kjellgren|1970|pp=40–44}}
|984 yds <br />(900&nbsp;m){{sfnp|Kjellgren|1970|pp=40–44}}
|3000 yds <br />(2700&nbsp;m){{sfnp|Kjellgren|1970|pp=40–44}}
|4.3 in <br />(11&nbsp;cm){{sfnp|Kjellgren|1970|pp=40–44}}
|12.6 in <br />(32&nbsp;cm){{NoteTag|Taken from the long-range dispersion firing test of the AK-47 assault rifle, U.S. Army Foreign Science and Technology Center. August 1969. "Just how mediocre? Two decades later, the U.S. Army would hold long-range firing tests with Kalashnikov variants, including three Soviet, two Chinese, and a Romanian model. At 300 meters, expert shooters at prone or bench rest positions had difficulty putting ten consecutive rounds on target. The testers then had the weapons fired from a cradle by a machine, which removed human error. At 300 meters, the ten-rounds group fired in this manner had a minimum dispersion of 17.5 inches, compared to the 12.6 inches with an M-16, the American assault rifle fielded in Vietnam as a reaction to the Kalashnikov's spread.{{sfnp|Chivers|2010|p=206}} }}
|}

[[File:NATO E-type Silhouette Target.PNG|thumb|upright=.5|NATO E-type silhouette target]]
{| class="wikitable" style="text-align: center;"
|- valign="top"
! colspan="11" |'''Single-shot hit-probability on crouching man (NATO E-type silhouette) target'''{{sfnp|Weaver|1990|p=87}}
|-
! rowspan="2" |Rifle
! rowspan="2" |Chambering
! colspan="9" |Hit-probability <small>(with no range estimation or aiming errors)</small>
|-
! <small>50 meters</small>
! <small>100 meters</small>
! <small>200 meters</small>
! <small>300 meters</small>
! <small>400 meters</small>
! <small>500 meters</small>
! <small>600 meters</small>
! <small>700 meters</small>
! <small>800 meters</small>
|-
|[[#XM16E1 and M16A1 (Colt Model 603)|M16A1]] <small>(1967)</small>
|[[5.56×45mm NATO|5.56×45&nbsp;mm NATO]] M193
|100%
|100%
|100%
|100%
|96%
|87%
|73%
|56%
|39%
|-
|[[#M16A2|M16A2]] <small>(1982)</small>
|[[5.56×45mm NATO|5.56×45&nbsp;mm NATO]] SS109/M855
|100%
|100%
|100%
|100%
|98%
|90%
|79%
|63%
|43%
|}

===Terminal ballistics===
The 5.56×45mm cartridge had several advantages over the 7.62×51mm NATO round used in the M14 rifle. It enabled each soldier to carry more ammunition and was easier to control during automatic or burst fire.{{sfnp|Crawford|2003|p=85}} 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.{{sfnmp|1a1=Courtney|1a2=Courtney|1y=2008|1p=4|2a1=McNab|2y=2002|2pp=108–109|3a1=Rose|3y=2008|3pp=375–376}}

{| class="wikitable" style="margin: 1em auto; text-align:center;"
|- valign="top"
! rowspan="2" |Rifle
! rowspan="2" |Caliber
! rowspan="2" |Cartridge
! colspan="7" |Penetration
|-
! <small>[[Ballistic gelatin]] <br />@ 10 meters</small>
! <small>[[Sandbag]]s <br />@ 100 meters</small>
! <small>3/4" pine boards <br />@ 100 meters</small>
! <small>Concrete building block <br />(one center rib)</small>
! <small>[[Steel helmet]]</small>
! <small>1.9mm steel <br />(14 gauge) <br />@ 100 meters</small>
! <small>4mm steel <br />(8 gauge) <br />+ layers of <br />Kevlar-29</small>
|-
!M16
|[[5.56×45mm|5.56×45&nbsp;mm]]
|M193
|<small>≈{{convert|14|in|cm|abbr=on}} <br />(bullet fragments <br />into smaller pieces){{sfnmp|1a1=Fackler|1y=2007|1p=1|2a1=Fackler|2y=2010|2p=3}}</small>
|<small>{{convert|4|in|cm|abbr=on}} <br />(complete bullet <br />disintegration){{sfnp|Defense Technical Information Center (DTIC)|1962|p=73}}</small>
|<small>8 boards <br />(bullet tumbled){{sfnp|Defense Technical Information Center (DTIC)|1962|p=73}}</small>
|<small>one side to 200&nbsp;m{{sfnp|Defense Technical Information Center (DTIC)|1962|p=73}}</small>
|<small>both sides to 300&nbsp;m <br />one side to 500&nbsp;m{{sfnp|Defense Technical Information Center (DTIC)|1962|p=73}}</small>
|<small>2 layers{{sfnp|Defense Technical Information Center (DTIC)|1962|p=73}}</small>
|<small>31 layers of Kevlar{{sfnp|Slepyan|Ayzenberg-Stepanenko|1998|pp=7}}</small>
|}

{{multiple image
| caption_align = center
| header_align = center
| align = right
| direction = vertical
| width = 300
| header = [[Wound ballistics|Wound profiles]] in ballistic gelatin <br /><small>Note: images are not to same scale</small>
| image1 = M16 5.56x45mm wound ballistics.gif
| alt1 = M16 5.56×45mm wound ballistics
| caption1 = M16 M193 5.56×45&nbsp;mm
| image2 = M16A2 M855 5.56X45mm NATO wound ballistics.gif
| alt2 = M16A2 M855 5.56×45mm wound ballistics
| caption2 = M16A2 SS109/M855 5.56×45&nbsp;mm NATO
}}
The original ammunition for the M16 was the 55-grain M193 cartridge. When fired from a {{cvt|20|in|mm}} barrel at ranges of up to {{convert|100|m|ft|sigfig=1|order=flip}}, the thin-jacketed lead-cored round traveled fast enough (above {{cvt|2900|ft/s|m/s}}) that the force of striking a human body would cause the round to yaw (or tumble) and fragment into about a dozen pieces of various sizes thus created wounds that were out of proportion to its caliber.{{sfnmp|1a1=Fackler|1y=2007|1p=1|2a1=Fackler|2y=2010|2p=3}} These wounds were so devastating that many considered the M16 to be an inhumane weapon.{{sfnmp|1a1=Hogg|1a2=Weeks|1y=1985|1pp=195-196|2a1=Prokosch|2y=1995|2p=1}} {{NoteTag|Those who consider the M16 inhumane include; the International Committee of the Red Cross, Austria, Argentina, Belgium, Bolivia, Bulgaria, Burundi, Cambodia, Cyprus, Germany, Ireland, Latvia, Lithuania, Luxembourg, Mauritius, Mexico, Romania, Samoa, Slovenia, Sweden, Switzerland, etc.{{sfnp|Parks|2010|pp=2}} }}As the 5.56&nbsp;mm round's velocity decreases, so does the number of fragments that it produces.{{sfnp|Ehrhart|2009|pp=25-26}} The 5.56&nbsp;mm round does not normally fragment at distances beyond 200 meters or at velocities below 2500&nbsp;ft/s, and its lethality becomes largely dependent on shot placement.{{sfnmp|1a1=Ehrhart|1y=2009|1pp=26-27|2a1=Fackler|2y=2007|2p=1}}

With the development of the M16A2, the new 62-grain M855 cartridge was adopted in 1983. The heavier bullet had more energy and was made with a steel core to penetrate Soviet [[body armor]]. However, this caused less fragmentation on impact and reduced effects against targets without armor, both of which lessened kinetic energy transfer and wounding ability.{{sfnp|''The New York Times, At War Blog'', November 12, 2009}} Some soldiers and Marines coped with this through training, with requirements to shoot vital areas three times to guarantee killing the target.{{sfnp|''The New York Times, At War Blog'', November 3, 2009}}

However, there have been repeated and consistent reports of the M855's inability to wound effectively (i.e., fragment) when fired from the short barreled M4 carbine (even at close ranges).{{sfnp|Ehrhart|2009|pp=27-28}} The M4's 14.5-in. barrel length reduces muzzle velocity to about 2900&nbsp;ft/s.{{sfnp|Colt, ''M4 5.56mm Carbine''|2003}} This reduced wounding ability is one reason that, despite the Army's transition to short-barrel M4s, the Marine Corps has decided to continue using the M16A4 with its 20-inch barrel as the 5.56×45mm M855 is largely dependent upon high velocity in order to wound effectively.{{sfnp|Ehrhart|2009|pp=27-28}}

In 2003, the U.S. Army contended that the lack of lethality of the 5.56×45mm was more a matter of perception than fact.{{sfnmp|1a1=Army Infantry Center|1y=2003|1p=8|2a1=Dean|2a2=LaFontaine|2y=2008|2p=3}} With good shot placement to the head and chest, the target was usually defeated without issue.{{sfnmp|1a1=Army Infantry Center|1y=2003|1p=8|2a1=Arvidsson|2y=2012|2p=769}} The majority of failures were the result of hitting the target in non-vital areas such as extremities.{{sfnp|Army Infantry Center|2003|p=8}} However, a minority of failures occurred in spite of multiple hits to the chest.{{sfnp|Army Infantry Center|2003|p=9}} In 2006, a study found that 20% of soldiers using the M4 carbine wanted more lethality or stopping power.{{sfnp|Rose|2008|pp=403–405}} In June 2010, the U.S. Army announced it began shipping its new 5.56&nbsp;mm, lead-free, M855A1 Enhanced Performance Round to active combat zones.{{sfnp|Woods|2010|p=35}} 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.{{sfnp|Woods|2010|p=35}}{{sfnp|''SCAS Hearings''|2010}} The U.S. Army has been impressed with the new M855A1 EPR round.{{sfnp|Slowik|2012|p=1}} A 7.62 NATO M80A1 EPR variant was also developed.{{sfnp|''The Firearm Blog'', 16 June 2016}}{{sfnp|''The Firearm Blog'', 23 July 2016}}

===Magazines===
[[File:Stanag mags.jpg|thumb|right|Vietnam War-era 20-round magazine (left) and current issue NATO STANAG 30-round magazine (right)]]
[[File:Improved M16 Magazine Follower.jpg|thumb|Improved tan colored M16 magazine follower]]

The M16's magazine was meant to be a lightweight, disposable item.{{sfnp|Bartocci|2011|p=1}} As such, it is made of pressed/stamped aluminum and was not designed to be durable.{{sfnp|Ehrhart|2009|p=39-44}} The M16 originally used a 20-round magazine which was later replaced by a bent 30-round design. As a result, the magazine follower tends to rock or tilt, causing malfunctions.{{sfnp|Bartocci|2011|p=1}} Many non-U.S. and commercial magazines have been developed to effectively mitigate these shortcomings, e.g., Heckler & Koch's all-stainless-steel "High-Reliability" magazine, [[Magpul]]'s polymer P-MAG and E-MAG, etc.{{sfnp|Ehrhart|2009|p=39-44}}{{sfnp|Bartocci|2011|p=1}}

Production of the 30-round magazine started late 1967 but did not fully replace the 20-round magazine until the mid-1970s.{{sfnp|Bartocci|2011|p=1}} Standard USGI aluminum 30-round M16 magazines weigh {{convert|0.11|kg|lb|abbr=on|order=flip}} empty and are {{convert|7.1|in|cm|abbr=out}} long.{{sfnp|Colt, ''M4 5.56mm Carbine''|2003}}{{NoteTag|Per Surefire" "Standard MILSPEC USGI 30-Round Magazine Specs: Height: 7.1" and Weight-Empty: 3.9 ounces"{{Sfnp|Crane|2010|p=1}}}} The newer plastic magazines are about a half-inch longer.{{sfnp|Crane|2010|p=1}} The newer steel magazines are about 0.5-inch longer and four ounces heavier.{{sfnp|Heckler & Koch|2013|p=1}} The M16's magazine has become the unofficial [[NATO]] [[STANAG magazine]] and is currently used by many Western nations, in numerous weapon systems.{{sfnmp|1a1=Dockery|1y=2007|1pp=125-126|2a1=Rottman|2y=2011|2pp=35-36}}

In 2009, the U.S. military began fielding an "improved magazine" identified by a tan-colored follower.{{sfnmp|1a1=PEO Soldier|1y=2009|1p=1|2a1=PEO Soldier|2y=2010|2p=1}} "The new follower incorporates an extended rear leg and modified bullet protrusion for improved round stacking and orientation. The self-leveling/anti-tilt follower minimizes jamming while a wider spring coil profile creates even force distribution. The performance gains have not added weight or cost to the magazines."{{sfnp|PEO Soldier|2010|p=1}}

In July 2016, the U.S. Army introduced another improvement, the new Enhanced Performance Magazine, which it says will result in a 300% increase in reliability in the M4 carbine. Developed by the [[United States Army Armament Research, Development and Engineering Center]] and the [[United States Army Research Laboratory|Army Research Laboratory]] in 2013, it is tan colored with blue follower to distinguish it from earlier, incompatible magazines.{{sfnp|'Soldier Systems Daily,'8 August 2016}}

===Muzzle devices===
Most M16 rifles have a barrel threaded in 1⁄2-28" threads to incorporate the use of a muzzle device such as a flash suppressor or [[sound suppressor]].{{sfnp|Sweeney|2007|p=67}} The initial flash suppressor design had three tines or prongs and was designed to preserve the shooter's night vision by disrupting the flash. Unfortunately it was prone to breakage and getting entangled in vegetation. The design was later changed to close the end to avoid this and became known as the "A1" or "bird cage" flash suppressor on the M16A1. Eventually on the M16A2 version of the rifle, the bottom port was closed to reduce muzzle climb and prevent dust from rising when the rifle was fired in the prone position.{{sfnp|Wieland|2011|p=200}} For these reasons, the U.S. military declared the A2 flash suppressor as a compensator or a muzzle brake; but it is more commonly known as the "GI" or "A2" flash suppressor.{{sfnp|Crawford|2003|p=85}}

The M16's [[Vortex Flash Hider]] weighs 3 ounces, is 2.25 inches long, and does not require a lock washer to attach to the barrel.{{sfnp|Hansen|2005|pp=28–32}} It was developed in 1984 and is one of the earliest privately designed muzzle devices. The U.S. military uses the Vortex Flash Hider on M4 carbines and M16 rifles.{{NoteTag|NATO Stock Number of NSN 1005-01-591-5825, PN 1001V{{sfnp|NATO Stock Number|2012}} }} A version of the Vortex has been adopted by the Canadian Military for the Colt Canada C8 CQB rifle.{{sfnp|Colt Canada|2012}} Other flash suppressors developed for the M16 include the Phantom Flash Suppressor by Yankee Hill Machine (YHM) and the KX-3 by [[Noveske Rifleworks]].{{sfnp|Sweeney|2007|pp=255–256}}

The threaded barrel allows sound suppressors with the same thread pattern to be installed directly to the barrel; however this can result in complications such as being unable to remove the suppressor from the barrel due to repeated firing on full auto or three-round burst.{{sfnp|Walker|2012|p=295}} A number of suppressor manufacturers have designed "direct-connect" sound suppressors which can be installed over an existing M16's flash suppressor as opposed to using the barrel's threads.{{sfnp|Walker|2012|p=295}}

===Grenade launchers and shotguns===
[[File:Loading M203 40 mm grenade launcher attached to an M16 rifle.jpg|right|thumb|Loading an [[M203 grenade launcher|M203 40&nbsp;mm grenade launcher]] attached to an M16A1 rifle with a practice round.]]

All current M16-type rifles can mount under-barrel 40&nbsp;mm grenade launchers, such as the M203 and [[M320 Grenade Launcher Module|M320]]. Both use the same [[40 mm grenade#40×46 mm LV (40 mm Low Velocity)|40×46mm LV]] grenades as the older, stand-alone [[M79 grenade launcher]]. The M16 can also mount under-barrel 12 gauge shotguns such as [[KAC Masterkey]] or the [[M26 Modular Accessory Shotgun System]].

===M234 launcher===
[[File:M234.png|thumb|upright=1.3|left|M234 riot control launcher]]
The [[M234 launcher|M234 riot control launcher]] is an M16-series rifle attachment firing an [[5.56×45mm NATO#Military cartridges|M755 blank round]]. The M234 mounts on the muzzle, bayonet lug, and front sight post of the M16. It fires either the M734 64&nbsp;mm kinetic riot control or the M742 64&nbsp;mm CSI riot control ring airfoil projectiles. The latter produces a 4 to 5-foot [[tear gas]] cloud on impact. The main advantage to using [[ring airfoil projectile]]s is that their design does not allow them to be thrown back by rioters with any real effect. The M234 is no longer used by U.S. forces. It has been replaced by the M203 grenade launcher and [[nonlethal]] ammunition.

===Bayonet===
The M16 is {{convert|44.25|in|mm}} long with an [[M7 bayonet]] attached.{{sfnp|Meehan|1985|p=32}} The M7 bayonet is based on earlier designs such as the [[M4 bayonet|M4]], [[M5 bayonet|M5]], & [[M6 bayonet|M6]] bayonets, all of which are direct descendants of the [[M3 fighting knife]] and have spear-point blade with a half sharpened secondary edge. The newer [[M9 bayonet]] has a clip-point blade with saw teeth along the spine and can be used as a multi-purpose knife and wire-cutter when combined with its scabbard. The current USMC [[OKC-3S bayonet]] bears a resemblance to the Marines' iconic [[Ka-Bar]] fighting knife with serrations near the handle.

===Bipod===
For use as an ad-hoc automatic rifle, the M16 and M16A1 could be equipped with the XM3 bipod, later standardized as the ''Bipod, M3'' (1966){{sfnp|Meehan|1985|p=4}} and ''Rifle Bipod M3'' (1983).{{sfnp|Hickerson|1991|pp=1-3}} Weighing only 0.6&nbsp;lb, the simple and non-adjustable bipod clamps to the barrel of the rifle to allow for supported fire.