What’s the Difference Between the M16A1 and M16A2?

You’ve heard the expression: If it ain’t broke, don’t fix it. Well, the M16A1 wasn’t exactly broke, but after its performance during the Vietnam War the United States Marines Corp requested the M16A1 be “fixed.” Perhaps it would more correct to say modified.

A U.S. Marine, from 1st Battalion, 3rd Marine Bravo Company, armed with an M16A2 rifle, on a squad rush at Schofield Army Barracks, Hawaii. Image: NARA

By 1979 when the request was received from the Corp, the modern battlefield was changing. Compared to the M14, the M16A1 was lightweight, had little recoil, and was easy to maneuver, but those who foresee the changing nature of warfare identified a need to change. The range in the anticipated new era of wars would be longer — 300 to 400 meters. In Vietnam, the average engagement distance was a short 25 meters.

101st Airborne Division soldier fires M16A1 rifle during Vietnam War combat operation in 1967 showing original A1 variant before M16A2 modifications were implemented. The M16A1 rifle features 20-inch barrel with 1:12 twist rate designed for lightweight 52-grain M193 ball ammunition at close engagement distances. Three-prong duck bill flash hider visible on A1 model was prone to catching vegetation and kicking up debris when shooting prone unlike later A2 bird cage design. Triangular handguard configuration on M16A1 used separate left and right components replaced by round interchangeable handguards on A2 variant. Short buttstock length of pull on A1 model was extended in M16A2 design for better shooting ergonomics and shoulder retention. Flip aperture rear sight on M16A1 offered two range settings but lacked adjustability compared to click-adjustable windage and elevation sights on M16A2. Full automatic selector setting on A1 fire control group was changed to three-round burst in M16A2 after Vietnam experience showed full auto wasted ammunition. Soldier demonstrates M16A1 rifle-length gas system and lightweight characteristics that made it maneuverable in jungle warfare versus heavier M14 rifle but revealed need for improvements incorporated into M16A2. — The M16A1 saw extensive combat use during the Vietnam War. Soldiers found the rifle lightweight and easy to maneuver compared to the heavier M14 it replaced. Image: NARA

A new bullet was being fielded, too. You might have heard of the M855? The bullet’s weight was increased for better long-range accuracy, and a steel tip was incorporated to punch through body armor. Those were two reasons the analysts had to revamp the A1. The boots on the ground, the Marines in particular, had reasons, too. A lot of reasons. It would seem the M16 needed to be a rifle with downrange performance more like the classic M14.

United States Marine Corps rifleman armed with M14 battle rifle in Vietnam War showing predecessor to M16A1 and M16A2 variants. The M14 rifle features excellent iron sights based on M1 Garand design superior to basic flip aperture on M16A1 and influencing adjustable sight development on M16A2. Traditional wood and steel construction on M14 contrasts with lightweight polymer materials used in M16A1 handguards and M16A2 Zytel stock components. Powerful .30-caliber chambering on M14 provides superior long-range performance compared to 5.56mm ammunition in M16A1 with 52-grain M193 and M16A2 with 62-grain M855 bullets. Heavier weight and recoil of M14 less suitable for jungle warfare at 25-meter engagement distances versus lightweight maneuverable M16A1 design. Marine Corps requested M16A2 modifications attempting to incorporate M14 characteristics including better downrange performance at 300 to 400 meters and improved sights. Robust M14 construction and traditional materials appealing for durability compared to early M16A1 components that lacked strength of later M16A2 polymer improvements. Vietnam scenario illustrates transition period from wood and steel M14 battle rifle to modern M16A1 lightweight design and subsequent M16A2 variant combining characteristics of both rifle systems. — The M14 served as the standard infantry rifle before being replaced by the M16A1 in Vietnam. Image: NARA

M16A2 Barrel Improvements

Let’s take barrel first. The A2 barrel is 20” long and has a 1:7-inch twist rate to better stabilize the heavier, longer bullets. The A1 also had a 20” barrel but with a 1:12-inch twist, which was fine for optimizing lighter, 52-gr. bullets like M1193 ball ammo, but has a harder time stabilizing heavier bullets. The barrel contour of the A2 is also beefed up forward of the handguard. Under the handguard, the barrel diameter is thin and exactly the same as the A1 to allow attaching the M203 grenade launcher.

United States Marine Corps guard armed with M16A1 rifle at Beirut International Airport bunker position shows A1 variant before M16A2 replacement in 1983. The M16A1 rifle features three-prong duck bill flash hider design that collected debris unlike enclosed bird cage flash suppressor on M16A2 variant. Triangular handguard system on A1 model used weaker material than reinforced polymer round handguards on M16A2 design. Short buttstock with smooth plate on M16A1 lacked extended length of pull and textured surface of M16A2 stock for better shoulder retention. Marine demonstrates M16A1 flip aperture rear sight with basic range settings compared to fully adjustable windage and elevation sights on A2 from 300 to 800 meters. Full-automatic fire selector on A1 fire control group visible differs from three-round burst capability on M16A2 requested by Marines. Barrel twist rate of 1:12 inches on M16A1 optimized for 52-grain M193 ammunition inadequate for heavier 62-grain M855 rounds requiring 1:7 twist on M16A2. Peacekeeping mission illustrates M16A1 service during transition period when Marine Corps requested modifications resulting in M16A2 variant with improved long-range accuracy and durability for 300 to 400 meter engagements. — A Marine armed with an M16A1 rifle stands guard in a bunker at Beirut International Airport. Image: NARA

The three-prong “duck bill” flash hider on the A1 had a habit of getting vegetation stuck in it as well as kicking up dirt or whatever was on the ground when shooting prone. A new A1 “bird cage” flash hider was enclosed so no debris could get caught in it, and slots were cut all around the circumference.

For the A2, the slots on the bottom of the flash hider were omitted so no dust is blasted when shooting prone. In addition, it acted like a muzzle brake to stifle muzzle rise during recoil. One thing that didn’t change was the rifle-length gas system, which makes both the A1 and A2 a soft shooter compared to a carbine- or even a mid-length gas system.

The M16A2 featured click-adjustable sights ranging from 300 to 800 meters. These improvements made the A2 variant more effective for precision shooting and long-range engagements. Image: NARA

A2 Has Better Sights

Being based upon the M1 Garand, the M14 was always renowned for its excellent iron sights. Conversely, one of the most distinct features of the M16A1 and the A2 is the built-in carry handle on the upper receiver that incorporates the rear sight. The A1 had a flip up aperture sight with two sizes of aperture, flipping the aperture also selected a range. Not the best set up for long range shooting.

The Marine Corp’s adaptation had the A2 incorporate a fully adjustable rear sight with click adjustments for both windage and elevation from 300 to 800 meters. The A2’s aperture also has two settings; the small aperture is for daylight and precision shooting, and the large aperture is meant for low-light scenarios.

M16A2 adjustable rear sight assembly showing major improvement over M16A1 flip aperture design with precision windage and elevation adjustments. Rear sight on A2 features click adjustments ranging from 300 to 800 meters compared to basic two-setting flip aperture on M16A1. Two aperture sizes on M16A2 rear sight include small aperture for daylight precision shooting and large aperture for low-light scenarios. Windage adjustment knob on A2 allows horizontal sight correction lacking on M16A1 flip aperture design. Elevation adjustment drum on M16A2 provides precise range settings from 300 to 800 meters versus simple flip mechanism on A1. Carry handle integrated rear sight on A2 influenced by excellent M14 iron sights requested by Marine Corps after Vietnam War experience. M16A2 sight improvements addressed long-range accuracy needs at 300 to 400 meter modern battlefield distances compared to 25-meter average Vietnam engagements with M16A1. Adjustable rear sight component illustrates key difference between M16A1 and M16A2 variants enhancing downrange performance and precision shooting capabilities similar to M14 characteristics. — The M16A2’s adjustable rear sight was a major improvement over the M16A1’s simple flip aperture.

The front sight on the A2 also changed to a flat faced post adjustable for elevation with four positions. The A1 had five settings and a round post, which created an aberration in certain light conditions causing groups to be off center.

Additional M16A2 Upgrades

The length of pull on A1 stocks was frankly too short, and also developed a reputation for a lack of durability. The A2 stock was made of stronger Zytel-type material that’s a glass filled thermoset polymer. The LOP was lengthened .62”, and the buttplate uses a toothy texture that didn’t slip out of your shoulder pocket like the A1 was apt to do. The hinged trap door to hold cleaning rods was retained.

United States Marine fires M16A1 rifle from fantail of USS Kitty Hawk aircraft carrier CV-63 showing A1 variant before M16A2 replacement in 1980s. M16A1 features three-prong duck bill flash hider that collected debris unlike enclosed bird cage flash suppressor on M16A2 variant. Triangular handguard system on A1 made from weaker material than reinforced polymer round handguards on M16A2 design. Short buttstock with smooth plate on M16A1 lacked extended length of pull and textured surface of M16A2 stock for better shoulder retention. Marine demonstrates M16A1 flip aperture rear sight with basic range settings compared to fully adjustable windage and elevation sights on A2 from 300 to 800 meters. Full-automatic fire selector on A1 visible differs from three-round burst capability on M16A2 requested by Marines for better accuracy. Barrel twist rate of 1:12 inches on M16A1 optimized for 52-grain M193 ammunition inadequate for heavier 62-grain M855 rounds requiring 1:7 twist on M16A2. Shipboard scenario illustrates M16A1 lightweight characteristics and rifle-length gas system with minimal recoil retained in M16A2 while incorporating enhanced sights, stronger materials, and improved long-range performance at 300 to 400 meter distances. — A Marine fires an M16A1 from the fantail of the aircraft carrier USS *Kitty Hawk* (CV 63). The A1 stock was noticeably shorter with a smooth buttplate that tended to slip from the shoulder pocket. Image: NARA

A big departure was the handguard, which is round, symmetrical and interchangeable on the A2. It was also made of a stronger polymer. The triangular A1 handguard used a left and right handguard. An interesting note is that soldiers more frequently dropped their A1s on the right side, which meant more right handguards, than left, needed to be in stock. The A2 pistol grip incorporated a finger hook which was designed to keep a user’s hand in place but in reality never really fit anyone’s hand that well.

The A2 changed to a three-round burst in lieu of full auto. The selector switch and the internals went from SAFE-SEMI-AUTO on the A1 to SAFE-SEMI-BURST on the A2. The auto setting on the A1 was found to waste a lot of ammo and was difficult to aim, especially under the stress and excitement of an engagement.

M16A2 rifle buttstock showing extended length of pull and textured buttplate improvements over shorter M16A1 stock design. A2 stock features toothy textured surface that prevents slippage from shoulder pocket unlike smooth buttplate on M16A1. Extended length of pull on M16A2 increased by 0.62 inches compared to frankly too short M16A1 stock for better shooting ergonomics. Stronger Zytel-type glass-filled thermoset polymer construction on A2 stock addresses durability concerns from weaker M16A1 material. Hinged trap door for cleaning rod storage retained from M16A1 design visible on M16A2 buttstock. Textured buttplate surface on A2 provides secure shoulder retention during recoil compared to slippery smooth plate on A1 variant. Stock modifications from M16A1 to M16A2 requested by Marine Corps after Vietnam War field experience identified need for better durability and fit. Component detail illustrates M16A2 improvements over M16A1 including longer stock, textured surface, and reinforced polymer material for enhanced reliability at 300 to 400 meter engagement distances. — The M16A2 buttstock featured an extended length of pull and textured buttplate that prevented slippage during firing. It was also made from stronger Zytel polymer to address durability complaints.

A brass deflector, basically a metal protrusion, was built into the upper receiver specifically for left-handed shooters, which is about 12 percent of the U.S. population. With the A1, hot brass is flung in front of a left-handed shooter’s face. This handy addition ensured the rifle was easy to use for both right- and left-handers.

Legacy of the M16A2

Unquestionably, the A2 variant was a huge step forward in making the M16 rifle more modern and effective. The M16 represented a sea change moment in firearms design, combining modern materials and manufacturing with a new “light and fast” approach to bullet design. In fact, the A2’s adaptations led to the development of the M4 Carbine so common these days.

The M16A2 served as the standard Marine Corps rifle. Its improved sights made it easier to teach proper marksmanship fundamentals than the M16A1. Image: DVIDS

However, there’s something to be said for the charms of the wood and steel M14. Was its heavier construction, traditional materials and .30-cal. chambering less than ideal for the jungles of Vietnam? Arguably, yes. Was its robust construction, excellent sights and powerful chambering an appealing and capable combination of characteristics in a service rifle? Unquestionably.

1st Infantry Division soldier carries M14 battle rifle in Vietnam War 1965 showing predecessor to M16A1 and M16A2 variants before transition to lightweight rifles. M14 features excellent iron sights based on M1 Garand design superior to basic flip aperture on M16A1 and influencing adjustable sight development on M16A2. Traditional wood stock and steel receiver construction on M14 contrasts with polymer handguards on M16A1 and Zytel stock on M16A2. Powerful .30-caliber chambering on M14 provides superior long-range performance compared to 5.56mm ammunition in M16A1 with 52-grain M193 and M16A2 with 62-grain M855 bullets. Heavier weight and recoil of M14 less suitable for Vietnam jungle warfare at 25-meter engagement distances versus lightweight maneuverable M16A1 design. Marine Corps later requested M16A2 modifications attempting to incorporate M14 characteristics including better downrange performance at 300 to 400 meters. Robust M14 construction and traditional materials appealing for durability compared to early M16A1 components that lacked strength of later M16A2 polymer improvements. Vietnam 1965 scenario illustrates early war period with M14 battle rifle before replacement by M16A1 and subsequent M16A2 variant combining characteristics of both rifle systems. — The M14 served as the primary infantry rifle in the early years of Vietnam before the M16A1 replaced it. The rifle’s weight and strong recoil made it less than ideal for jungle warfare. Image: NARA

There’s an argument to be made that the USMC was trying to turn the M16A1 — through its A2 modifications to the rifles’ construction, operation, long range performance, and durability — into something more like the old M14.

Whatever the motivations, the M16A2 stands as benchmark in modern rifle design and is now as iconic as it is classic.

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