Why the M16 and M4 Haven’t Been Replaced: A History


A US Marine aims an M16A4 rifle during a training event.

The iconic M16 rifle and M4 carbine, standard issue in the United States military, are some of the most well-known weapons today. Lightweight and accurate, the M16 was the military’s standard infantry rifle for over fifty years before recently handing the title to its smaller sibling, the M4. This article will attempt to explain how the M16 design has remained in service for so long and survived multiple attempts at replacement.

The M16 and M4: a brief history

The M16 design derives from the AR-10, an extraordinarily light experimental rifle chambered in 7.62x51mm NATO. The AR-10 was envisioned as a replacement for the M1 Garand rifle of World War II and featured a number of revolutionary features, such as a composite stock, an aluminum alloy receiver, and an integrated flash suppressor/recoil compensator. While advanced for its time, the design was passed up in favor of the more conventional M14.

Years later, in Vietnam, the Army was forced to rethink its approach to small arms. Because of the tremendous recoil produced by the full-power 7.62 NATO round, the M14 was virtually impossible to fire accurately in automatic. The M2 carbine, a lighter weapon firing the.30 Carbine round, was usable in automatic but lacked range and power.


From left to right: 7.62 NATO, 5.56 NATO, AA battery.

The solution was to create an intermediate round, 5.56x45mm NATO. Superior to .30 Carbine in terms of muzzle energy and range while still “weak” enough to be fired rapidly, 5.56 NATO provided a middle ground that was suitable for the average rifleman. Since 5.56 NATO is smaller and lighter than 7.62 NATO, it allows troops to carry far more ammunition as well. The AR-10 prototype was modified to create the AR-15, which retained most of the AR-10’s features and fired 5.56 NATO. Initial reactions were positive among test groups in Vietnam, so the AR-15 was accepted into service and designated the M16. Nowadays, “AR-15” generally refers to semi-automatic versions of the gun which are sold to civilians.

However, wide scale adoption of the M16 revealed some issues. The largest problem cited by troops in Vietnam was poor reliability, an issue which continues to afflict the M16 into the present day. To explain this, a brief overview of automatic firing mechanisms is necessary.


An animation depicting a piston-type mechanism. Note that the hot gasses are directed into the piston but do not enter the receiver (at left).

The AK-47 and many other automatic weapons use a piston to cycle the bolt. This means that the hot gasses produced by firing are vented into a small chamber where they push a piston, providing the force to extract the old round and chamber a new one. This type of system tends to be heavier and can disrupt aim, but is also very reliable since the dirty gasses produced during firing are restricted to the piston and do not directly enter the receiver. Also, piston systems are less likely to be infiltrated by dirt, mud, dust, etc.


An animation depicting a direct impingement-type mechanism. Note that the hot gasses are directed back into the receiver itself.

The M16 (and its derivatives such as the M4) use direct impingement, which eliminates the piston and instead vents the gasses back into the receiver where they cycle the bolt directly. Foregoing the piston reduces weight and simplifies the mechanism, but it means that the chemical-filled gasses can foul up the action and cause failures. Direct impingement mechanisms also tend to be relatively susceptible to contamination by fine particles of dirt, mud, dust, etc.

The original M16 suffered especially poor reliability, but the M16A1 variant added chrome chamber plating and other improvements which addressed the problem to an extent.  However, in Iraq and Afghanistan, questions were again raised about the rifle’s performance. A 2007 survey found that 20% of infantrymen who used their M16s in combat suffered stoppages which caused an “[i]nability to engage [the] target with [the] weapon during a significant portion or entire firefight.” An even larger 80% of users reported experiencing minor jams.

Another interesting finding of the survey: many users found the M16 too large and unwieldy. This was a sentiment echoed so frequently that the military decided to adopt the M4 carbine as the standard infantry rifle. The M4 is basically an M16 with modifications such as a smaller barrel and collapsible stock, making it more nimble in close quarters combat but largely the same to operate and maintain.

Despite the stoppage issues, the M16 and M4 were actually quite popular — 75% of M16 users and 89% of M4 users reported being satisfied with their weapon. This was higher than the satisfaction rates for the M9 pistol and the M249 light machinegun, the two other weapons included in the survey.

Failed efforts to replace the M16 and M4

Over the five-decade history of the M16 and M4’s service, there have been numerous attempts to develop a replacement, often attempting to incorporate cutting-edge technology.

The first major effort was a multi-decade program called Special Purpose Individual Weapon (SPIW), the first phase of which actually preceded the adoption of the M16. The program was aimed at achieving a breakthrough in small arms technology by exploring alternative lightweight ammunitions, mainly flechette rounds, which are essentially small metal darts. Because of their compact size, many such rounds could be stored in a single magazine, and cyclic fire rates exceeded 2,000 rounds per minute, significantly more than the <1000 round cyclic rate of conventional automatic rifles. Nevertheless, the SPIW entries suffered a range of issues, from excessive vibrations to bulkiness and poor reliability, so the program was terminated.

Out of its ashes rose the Advanced Combat Rifle (ACR) program, which was aimed at increasing the accuracy of riflemen in stressful conditions by delivering multiple rounds with each trigger pull. The study concluded that a significant improvement in accuracy could not be achieved by kinetic (non-explosive) weapons.


A model of the XM29 OICW.

Informed by that conclusion, the military decided smart airburst grenades could be the solution, allowing soldiers to achieve kills without the need to directly hit their target. To this end, the military began the Objective Infantry Combat Weapon (OICW) program, which endeavored to produce a lightweight semi-automatic grenade launcher fused with a conventional automatic gun. This effort resulted in the XM29, a futuristic blend of a G36K-derived assault carbine chambered in 5.56 NATO and a semi-automatic grenade launcher firing 20mm airburst rounds. An integrated targeting system provided magnified optics and fire control for the grenades. While a compelling concept, the weapon had serious issues. For one, it was unacceptably heavy, clocking in at 8.2kg, more than double the 3.3kg of the M16 it was designed to replace. Ergonomics were also very poor, and the carbine portion could not be detached from the weapon, as it lacked a stock. Furthermore, both the carbine component and the grenade launcher both had poor lethality, the former due to barrel length and the latter due to the grenade’s small explosive payload. As a result, the program was terminated — in a way.


US Army officials fire XM8s.

Rather than canceling OICW outright, the Army chose to develop each portion of the XM29 separately, spawning the XM8 rifle and the XM25 grenade launcher. The XM8 was a carbine-sized weapon which included an integrated sight as well as a gas piston action for reliability. However, replacing every M16 and M4 would have been massively expensive, and the XM8 did not offer large enough improvements to justify such an undertaking.

These failures essentially marked the end of the effort to replace the M16 with futuristic and highly ambitious alternatives. Future programs would focus on simply achieving improvements in reliability and handling, rather than a revolution in small arms technology.

Two FN SCAR rifles — the top chambered in 5.56 NATO and the bottom in 7.62 NATO. Image by Mitch Barrie.

The first such effort was the Special Forces Combat Assault Rifle (SCAR) program, initiated by Special Operations Command (SOCOM) to supplement their M16 and M4 with a reliable, modular alternative. The competition was ultimately won by FN, whose entry came to be known as the FN SCAR. SOCOM eventually opted not to replace all existing weapons with the FN SCAR, instead purchasing a smaller number of 7.62 NATO SCAR-H rifles with 5.56 NATO conversion kits. SOCOM also purchased a number of HK416s, which share many parts with the M16 and M4 but incorporate a short-stroke gas piston system for enhanced reliability.

In 2007 the Army conducted a dust test, in which 6,000 rounds were fired, to shed light on the reliability of a number of available weapons. The M4 suffered 882 stoppages, the HK416: 233, the FN SCAR: 226, and the XM8: 127. On top of that, the M4 was subject to lubrication and maintenance beyond normal levels. During the test, all M4s used all required barrel replacements, which none of the other weapons needed. Nevertheless, the military saw no need to replace the M4 at the time.

A few years later, the Army initiated a competition called Individual Carbine was begun to explore alternatives to the M4, but it was abruptly canceled shortly thereafter. The weapon manufacturers who participated were unsurprised, stating that they did not actually expect the military to replace the M4 through Individual Carbine.

Recently, concerns have been mounting that the 5.56 NATO round lacks knockdown power. A number of new cartridges in between 5.56 and 7.62 NATO in power, such as 6.5mm Grendel and 6.8mm Remington SPC, have been developed by the private sector. The continued improvement of body armor, as well as the usage of high-powered 7.62x39mm rounds by AK-wielding insurgents, is pushing the US Army to test some of these alternatives. If the decision is made to abandon 5.56 NATO, a new rifle might also be in the cards, although the M16 and M4 could be modified for the job. Of course, abandoning 5.56 NATO would bring significant financial and logistical challenges, as the round is used not only by all branches of the US military but also all other NATO militaries. Thus, it seems rather unlikely anything will come of the tests, but there is a possibility.

Closing thoughts

So, why have the M16 and now the M4 served for so long? The answer depends in part on which replacement program one looks at. High-tech efforts such as SPIW, ACR, and OICW failed because of their complexity and ambition. Like many other military programs, their underlying ideas were compelling but the highly experimental nature of the programs led to engineering troubles and rising costs.

The other solutions, including simply replacing the M4 with a more reliable equivalent such as the HK416, failed for the opposite reason. While reliability is important, replacing every single automatic rifle in the military is a tremendous undertaking. Not only do the guns have to be purchased, but every soldier has to be re-trained, procedures need to be re-written, etc. Such efforts are usually made only when there is a pressing need, such as the M14’s uselessness in automatic fire and the heaviness of its ammunition. While increased jamming in adverse conditions is a problem, military officials have not deemed it pressing enough to warrant a comprehensive upgrade.

Another factor is that the M16 and M4 are simply well-liked weapons. The military’s own surveys demonstrate this, as discussed above, as does the AR-15 platform’s massive popularity in the civilian shooting world. Like everything, firearms engineering is a tradeoff. The M16 and M4 may have drawbacks, but they compensate with light weight, accuracy, and ease of use, and users appreciate these features.

Of course, there will eventually be a replacement. At some point, a new development in technology, such as smart grenades or directed energy, could lead to a new standard for infantry weapons. Or, the AR-15 platform could gradually fall behind the newest private sector designs until the military finally caves. But, for now at least, the venerable M16 and M4 are here to stay.

About the Author

Alex Hempel
I am the owner of the site and the author of all content. You can reach me at alexhempel2012@gmail.com.

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