Since its introduction in 2009, the Marine Corps’ M27 Infantry Automatic Rifle has proven itself as an effective support weapon that offers more firepower and range than the Corps’ other squad level weapons, the M4 Carbine and M16A4 Rifle. This good reception has led many within the Corps to reach an obvious conclusion: The USMC should simply replace their M4s and M16A4s in the Infantry Battalions with M27 IARs. This idea gained so much traction that the USMC has experimented with arming an entire battalion with IARs, and even released an RFI to the industry for 11,000 more IARs.

To explore this idea further, I sat down with an anonymous small arms subject matter expert to discuss the M27 IAR, its potential as an infantry weapon, and other possibilities for the next Marine Corps infantry weapon. Together, we came to a surprising conclusion: The M27 IAR is not the right infantry rifle for the Marine Corps. In this article, I will lay out the factors that led us to this conclusion, and suggest a potentially superior alternative.

Before we talk about why the M27 would not be the right choice, though, let us acknowledge what it is that the M27 brings to the table as a fighting weapon. To start, versus the M4 Carbine and M16A4 Rifle, the M27 IAR has a superior barrel and handguard system. The M4 Carbine’s barrel is made of the same 4150 steel (a low grade chome-molybdenium steel) that has been the military standard since World War II. In the M4, this steel is button rifled, heat treated to between 277 and 331 Brinell (approximately 28-36 Rockwell C) and then chrome-lined. With the M27, in contrast, the barrel is made of high chromium content Aubert & Duval chrome-molybdenum-vanadium steel, cold hammer forged to give a tapered bore, and heat treated to 41 Rockwell C, then chrome lined.

These material characteristics, especially the heat treat, give the barrel exceptional life and accuracy, and it is easily this factor which is most attractive about the M27. In addition, instead of the M16A4’s and M4’s classic delta-ring system, the M27 uses a greatly improved barrel nut and handguard system that interfaces via a groove with a free-floated handguard unit, improving barrel harmonics. For the Marine Corps, the ability to make accurate hits from any position – whether shot slung, from a barricade, off a bipod or pack, etc – without substantial POI shift thanks to a free-floated barrel unit is value-added for each rifleman.

Together, these features mean, simply: The M27 is more accurate, stays accurate longer, and has a longer barrel life than either the M4 Carbine or the M16A4 Rifle. A rifle that maintains its accuracy and precision longer is one that gives more hits, better suppression through closer misses, and overall greater combat effect.

Given then that the M27 offers an improvement over the existing weapons, why shouldn’t the Marine Corps adopt it? The reason at its heart lies in the fact that the M27 is still very much a weapon that was developed in the early 2000s, and selected based on a 2007 RFI. It is not that the M27 is a poor weapon, but rather that, in the ten years since the Infantry Automatic Rifle program was made public, substantial commercial off the shelf (COTS) improvements have been introduced that could provide a weapon of equal or greater capability to the M27, but at lower cost and lower weight. Even better, by upgrading the M4 Carbine various engineering shortcomings inherent to the M27 can be avoided entirely.

This sort of improvement could be achieved via a depot level or upper receiver group upgrade program to the existing M4, which opens a number of procurement avenues for the Marine Corps that otherwise would be closed if the M27 were sought instead. Sole-sourcing the M27 as a standard infantry weapon virtually constitutes doing it backwards: Instead of learning from the Corps’ experience with the IAR and implementing the best possible fleet solution to provide the required capability to the Infantry, they would simply be sticking a ring on their first major crush. The M27 does pave the way forward, but it itself is already dated technology. A more sound approach would be to leverage the competitive environment to procure something better, lighter, and cheaper.

One area where a new rifle might improve over the M27 is in the bolt and gas system configuration. Our source told TFB that with M855A1, the M27 achieved an average bolt life (measured to the first lug shearing off) in official high round count tests of 6,000-7,000 rounds, and a barrel life (measured to 200 ft/s velocity loss) of 9,000-10,000 rounds. In those same tests, the M4A1 achieved a barrel life of just 7,000 rounds due to its inferior steel and construction (though the carbines continued to group well for thousands more rounds). However, the M4A1 Carbine beat the M27 in bolt life, with an average of 9,000 rounds before lug shear. One M4 bolt even went 13,000 rounds before its first lug sheared! As I will elaborate below, this disparity in bolt life is the result of something peculiar and inherent to the M27’s design, and it would therefore take considerable time, effort and money via extensive equipment change proposals (ECPs) to rectify the problem. For the M4 Carbine, however, even better bolt life than current could be achieved via simple drop-in upgrades and improvements at very low cost.

The reason for the M27’s lackluster bolt life has to do with its gas system. From its inception, the HK416 (the family to which the M27 belongs) was designed as a short-barreled rifle, an industry-driven effort to create an alternative to the early Mk.18 Close Quarters Battle Receiver. To achieve the reliability needed, Heckler & Koch dramatically increased the gas flow to the HK416’s short-stroke piston. This improved reliability with the 10.5″ barrels and H&K “Navy” steel magazines of the original requirement. Since then, the HK416 has greeted the wider market largely as a carbine-length-barreled true assault rifle, but it has always retained the original gas system design of the short barreled variant. This has led to an “overgassed” situation, resulting in high cyclic rates and increased wear, a situation which is exacerbated by the rifle’s short-stroke piston-driven operating rod system.

In detail, the HK416’s operating rod transmits the kinetic energy of the expanding gases to movement in the moving parts group virtually immediately after gas is tapped into the gas block, rather than allowing the gas to expand via a tube as in Stoner direct impingement. This means that if gas is not aggressively vented early (such as via the uncorking of the bullet from a short 10.5″ barrel), then the system is subjected to considerable excess thrust from the high pressure gases still acting on the piston as the bullet travels down the remainder of the barrel’s length. In the longer-barreled HK416 models like the M27, this causes the bolt to unlock prematurely at a point when the pressure is still high, and unlike direct impingement AR-15s there is no forward gas pressure on the bolt stem helping to unload the bolt lugs off the extension.

All this together results in higher lug stresses and lower bolt life for the M27 versus the M4 Carbine, in addition to higher cyclic rates which require more tightly sprung and more durable magazines which can keep up with the fast moving bolt group. Aggravating this problem, the M27 manual requires that, unlike with the M4, bolt groups be replaced as a unit. This means that when the bolt shears a lug, the entire bolt and carrier assembly must be removed and replaced with a new unit. Ideally, a new upper receiver for the M4 Carbine would incorporate a barrel of similar construction and materials to the HK416, but utilize a mid-length direct impingement gas system to reduce bolt fatigue, increase dwell time, reduce bolt velocity, and dramatically improve cycling characteristics overall. Further, it could incorporate new bolt treatment techniques that could dramatically improve part life far beyond the current norm, substantially increasing the maintenance interval for this part.

Another one of the chief advantages of the M27 IAR versus its stablemates is its fire control group. In lieu of the M4 and M16A4’s trigger groups which provide safe, semiautomatic, and 3 round burst settings, the M27 IAR’s fire control replaces the burst function with a fully automatic provision which allows the rifle to be fired for extended bursts. This not only provides an increase in firepower, it also improves the consistency of trigger pull and prevents malfunctions that can be induced by the burst mechanism of the M4 or M16A4. However, as part of this package, the trigger group of the M27 also provides a reduced weight pull primarily through a reduced power hammer spring. This feature carries with it the side effects of higher bolt carrier speeds (via less hammer delaying effect) and reduced disconnector life.

Since the selection of the M27 IAR, newer select-fire trigger units for the AR-15 family have been introduced which improve upon the consistency and function of previous triggers primarily through greater uniformity and precision of their sear surfaces, rather than reduced hammer weight. Perhaps the standard bearer of these triggers has been the Geissele Super Select-Fire (SSF) trigger, which already has a national stock number (NSN) and is drop-in compatible with the USMC’s existing M4 and M16A4 fleet. That one modification, together with other upgrades, would bring a degree of precision and firepower to those weapons equal or greater than the M27 IAR.

The free-float rail of the M27 is one of the most visually obvious differences between it and its legacy stablemates, and also one of its key advantages as previously mentioned. However, even it is dated by today’s standards, thanks to the advent of negative-space mounting systems (which were in fact pioneered by Heckler & Koch, but that is another story) like M-LOK. The M27’s Picatinny-type free-float rail is heavier, more damage-prone, and allows less air circulation than a modern negative-footprint rail. In addition, the clever grooved barrel nut system of the M27 possesses some limitations. First, it is made from steel tubing, and consequently the finished products are slightly out of round, hampering rigidity and rail alignment.

This rail rigidity is important for night operations when using a rail-mounted laser aiming device: A 10 MOA shift in the rail’s position results in a target miss at 200m. Also, the steel construction of the nut and heavy barrel of the M27 – though one of the rifle’s key features – mean the rifle takes a long time to overheat, but also a long time to cool off as well. Further, the steel barrel nut wears the threads on the receiver relatively quickly as barrels are taken on and off for maintenance, meaning receivers must be replaced more regularly. A barrel system using an improved aluminum barrel nut would alleviate all of these issues. Such barrel nuts are already being produced via the commercial market that are more precise thanks to manufacturing via grinding or or lathing. The aluminum construction would give three improvements: First, it would act as a heat sink, conducting and dissipating heat more quickly than a steel barrel nut.

Second, it would reduce wear on the receiver threads, improving receiver life. And third, it would reduce the weight of the barrel nut assembly and potentially also allow a slight reduction in the weight of the barrel itself thanks to improved cooling characteristics. A new barrel nut of this type could also be paired with a modern lightweight rail design, for example a 13″ rail with M-LOK slots at 3, 6, and 9 o’clock, and a full length 1913 rail at the noon position. A rail like this would not only provide flexible mounting options, but also reduce the thermal and IR signature of the rifle versus the M27 (which, besides having a barrel that stays hot for longer, and a conductive barrel nut, also vents gas out of the front of its gas block).

Given recognition of these shortcomings of the M27 when compared to more recent developments, what does the ideal USMC infantry rifle look like? It would utilize a 14.5″ medium-heavy barrel of comparable construction and metallurgy to that of the M27, with an improved barrel nut design coupled to a modern lightweight handguard. It would incorporate a true “military match grade” select fire trigger system that would give the rifleman precision capability equal to or better than that provided by the M27 IAR, with improved reliability and durability. It would feature a midlength direct impingement gas system which would be lighter, more reliable, and lower signature than that of the M27, while giving improved bolt and moving parts group life. Overall, such a rifle would provide the Marine rifleman with superior accuracy and firepower versus the M4 Carbine and M16A4 Rifle, while improving reliability, durability, and longevity versus the M27 and possibly even the M4 Carbine and M16A4 Rifle as well.

Most compelling, however, is the fact that such a weapon system is achievable through commercial off-the-shelf and non-developmental upgrades to the existing M4 Carbine and M16A4 Rifle. Instead of the M27, which can only be procured via a new sole-source contract that would be highly vulnerable to protest from competing firms, the USMC could leverage the competitive market to provide a superior solution now, which could then be procured in any number of ways. Rifles and carbines already in inventory could be upgraded through arsenal overhauls with new barrels, triggers, and rails, or alternately a new upper receiver group could be solicited. Multiple procurement paths means less time and effort wasted before the right solution reaches the hands of Marines downrange, and it allows the Corps to leverage the entire industry to provide the most competitive, lowest cost solution, instead of making it dependent on a single company.


What does the next USMC rifle look like? A bit like this, perhaps. 14.5″ barrel, 13″ lightweight handguard and barrel nut, military match grade trigger, and built off existing M4 and M16 receivers.

In brief, the M27 is a fine weapon that undoubtedly provides an improvement in capability versus existing USMC small arms. It is also, however, already a dated system representing a decade-old state of the art that has been surpassed by more refined commercial improvements to the AR-15 family, which includes the M4 Carbine and M16A4 Rifle. The M27 itself is also not the best host candidate for upgrade, since it is fundamentally hampered by its gas system design. Since the M27’s selection, off-the-shelf or otherwise non-developmental solutions have arisen which leverage both existing M4 and M16A4 receivers and the commercial market to provide potentially equal or greater capability at lower cost and weight.

Implementing these improved solutions instead of a fleet-wide adoption of the M27 would also expand the number of avenues of procurement for the Marine Corps, speeding delivery of much needed upgraded weapons to Marines at the front. In short, an M4/M16A4 upgrade program would provide a more capable, lighter, and cheaper solution that is quicker and easier to procure, and which wouldn’t tie the Corps to any one company for its future rifle needs.





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