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.

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 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.

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.

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.

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.

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

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.

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

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.

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