The Army Is Working On A Mini Assault Rifle That Performs Like An M4, But Is Half As Big

The Army says the core technology could help in the development of lighter, smaller rifles and machine guns, too.

byJoseph Trevithick|
M16/AR15/M4 Pattern photo


U.S. Army researchers are working to develop a personal defense weapon that offers the same muzzle velocity as a standard M4 carbine in a package that's half as long, half the weight, and uses smaller, lighter ammunition. The service's engineers say that the design features they are working with to achieve these results are readily scalable and could offer a path to lighter weight rifles and machine guns in the future, too.

In March 2019, the Army Research Laboratory (ARL), situated at Aberdeen Proving Ground in Maryland, test-fired an experimental design with a 10 inch-long barrel using ammunition with just a single gram of gunpowder, according to a story from TechLink. The gun demonstrated a muzzle velocity of more than 2,900 feet per second. An M4 firing standard 5.56x45mm M855A1 ammunition, which has around one and a half times as much powder, has an average muzzle velocity of around 2,970 feet per second.

“The goal is to get rifle-like velocities out of a very small weapon that is high capacity, that’s either adaptable for room-clearing or confined spaces,” said Zac Wingard, a mechanical engineer at ARL told TechLink. TechLink, part of the Montana State University’s Office of Research and Economic Development, helps the Department of Defense license technology it develops in-house to private firms who can then further develop it into commercially viable, mass-producible products.

Much of the technology behind ARL's prototype miniature assault rifle is rooted in well-established concepts. The biggest issue when designing any gun to fire higher velocity ammunition, is the need to safely contain the pressure when the gunpowder inside the cartridge detonates. If the firearm isn't strong enough to handle these forces, it could break, potentially so severely as to injure the shooter. 

The video below gives a detailed overview of the inner workings of an AR-15/M16 pattern gun, which gives a good general sense of the various physical forces at play inside a typical modern rifle. Other designs differ in their exact internal mechanisms, but the basic sequence of operation – chambering a round, firing that round, extracting and ejecting the spent cartridge case, and resetting the system to fire again – is effectively the same.

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Historically, the main way of mitigating this has been to reinforce the breech face and chamber, which seal the cartridge in place before it goes off, as well as other components. This, however, adds weight and bulk that does not necessarily lend itself to compact, lightweight firearms.

"The powder used now in most ammunitions can be tweaked, so it runs at a higher pressure, but the guns can’t handle it,” Alex Michlin, another ARL engineer, explained to TechLink. "That’s why we designed the new breech, so we can take existing propellant and turn the knob all the way up to 11."

Michlin holds the patent for the new breach design, which screws securely into place as the weapon cycles, helping distribute the force along a greater area and increasing the pressure the gun can handle without adding significant extra weight. It's not clear, but it does appear that this design would add some extra length since the mechanism needs more distance to lock and unlock during firing than a more traditional system. 

This might not be a serious issue if designers can use a shorter barrel and still hit the desired performance targets. A bullpup design or a flush, top-loading magazine could also help in reducing the gun's profile.

Drawings from Michlin's patent showing the screw locking design and how it works., USTPO

Another issue is that higher pressures cause metal cartridge cases to expand more, increasing the chance that they could get stuck in the breech and jam the gun, especially during fully automatic firing. Michlin patented a second component that uses tapered wedges that grip the sides of the cartridge cases, reportedly requiring 50 percent less energy to do so in a more traditional system that yanks them out backward by their base. This could actually further improve the reliability of the gun, since it requires less energy overall to properly cycle the action after each shot.

A diagram of the tapered wedges that extract the cartridge cases., ARL

Lastly, the experimental gun uses a special, tapered barrel that gets narrower right at the end, physically squeezing the projectile into a narrower profile, which gives it an extra boost in velocity as it pushes out the end. This concept, commonly known as a "squeeze bore," dates back to Nazi Germany in the 1930s. The Army, among others, also tested various guns using these types of barrels, ranging from small arms to light cannons, for decades afterward, but never fielded any of the designs.

A diagram showing the general barrel configuration ARL has been experimenting with, which features a traditional, rifled section and then a smooth, tapered portion., ARL

The end result is a weapon that can remain compact, but keep velocities up closer to a standard carbine or rifle. This would also help ensure the shorter-barrel design has adequate armor penetration capabilities, which is something the Army is becoming increasingly concerned about, in general, as potential opponents, such as Russia and China, field new and improved body armor.

If ARL's design is viable outside the laboratory, it might lead to a revival of the squeeze bore concept and could have a significant impact on personal defense weapon designs. Over the years, various companies have worked on personal defense weapons using highly specialized ammunition to get as close to rifle-like performance as possible at relatively short ranges into extremely compact packages. The 4.7mm Heckler and Koch MP7 –  as seen at the top of this story – and 5.7mm FN P90 are perhaps the best-known examples and are in service with the U.S. Navy's SEAL Team Six and the U.S. Secret Service, respectively, among other users.

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TechLink noted that the P90 also has a 10-inch barrel, but only has a muzzle velocity of around 2,350 feet per second. ARL's gun also reportedly has a 50-round magazine capacity like this Belgian personal defense weapon. But without knowing the exact specifics of ARL's new ammunition or its experimental gun, it's almost impossible to make a real direct comparison between the two. 

The Army did recently hire the Defense Ordnance Technology  Consortium, another public-private partnership, to facilitate the development of a new personal defense weapon, according to the Spring 2019 issue of Army AL&T, an official magazine of the Army Acquisition Support Center. However, it is unclear if this is at all related to the experimental ARL program. AL&T included a picture of Sig Sauer's Rattler, a gun that U.S. Special Operations Command is testing and that you can read about in more detail here, as a representative design. This gun does not use any of the technology that ARL says it is working with for its experimental design.

A picture of a Sig Sauer Rattler from Army AL&T magazine., via US Army

ARL also told TechLink that the design it was working on is scalable into larger packages that could serve as smaller, lighter substitutes for full-size rifles or machine guns. Another prototype gun with a 24-inch barrel produced muzzle velocities between 4,600 and 5,750 feet per second. A full-size M16 service rifle, which has a 20-inch barrel, has an average muzzle velocity of around 3,150 feet per second with standard 5.56x45mm M855A1 ammunition.

If these guns can also use lighter ammunition, it could give troops additional firepower, but with reduced weight and bulk. This would be especially valuable for regular units conducting extended dismounted patrols or special operations forces that might find themselves operating well beyond traditional supply lines for protracted periods.

It's not clear when troops might actually find themselves armed with guns based on ARL's current research. "This effort is about accelerating technology transition from the laboratory prototype and integrating into other programs," T'Jae Ellis, an ARL spokesperson, told Task & Purpose, adding that the project was not directly unrelated to the Army's development of a family of rifles and squad automatic weapons that will use a new 6.8mm cartridge.

But, at some point in the future, Army soldiers may find themselves on the battlefield carrying an entirely new suite of compact, lighter weight guns based on the prototypes that ARL is experimenting with now. 

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