The primary long arm of the Martian Congressional Republic Navy (MCRN) and the Martian Marine Corps (MMC) is a bullpup style assault rifle first seen in Season 1 Episode 3. The weapon appears regularly throughout the series with few modifications .
Thanks to its bullpup design, the full weapon is only ~65 cm long, which means it is just over two thirds of the size of the UN Assault Rifle. This makes the weapon more suited for use in confined spaces compared to the UN rifle, which also makes it a natural choice for Martian forces.
In the books Martian weapons are described as being able to fire both standard and recoilless ammunition. This may be the reason why both the Martian assault rifle and the Martian service pistol appear to have two barrels – one over the other. One barrel is likely intended for standard ammunition and one for recoilless.
The standard ammunition is believed to be 6.25 mm incendiary caseless ammunition. In the show the Rocinante is seen carrying this type of ammunition in its armory, salvaged from the Kittur Chennamma, presumably intended for assault rifles as neither ship appears to regularly carry marines equipped with Goliath power armor.
As the name suggests, caseless ammunition does not have a case behind the bullet/projectile to hold the necessary gunpowder and the associated igniter at its base (known as a primer.) Instead all of this is self contained, and everything except the bullet is consumed by the explosion when the gun is fired. The photo seen here (sourced here) demonstrates the difference between standard cased and caseless ammunition. The left cased cartridge is equivalent to the ammunition used in most NATO assault rifles in the early 21st century. The right one was designed for the German Heckler & Koch G11 experimental assault rifle, which was developed at the end of the Cold War as a potential replacement for rifles using cased ammuntion. Here the bullet is housed inside the propellent, and only has a thin white plastic layer at the top. Though the G11-project was scrapped after German unification at the end of the Cold War (when the German government didn’t want to spend money on a fancy new gun and instead went for the more conventional G36), it did prove that caseless ammunition was possible in an automatic weapon.
Eliminating the case however makes a lot of sense for space operations, as it not only makes the ammunition lighter and smaller, but also avoids the problem of dealing with hot, spent cases flying out of the weapon.
In addition to a barrel suitable for caseless ammunition, Martian assault rifles are likely also fitted with a second low-recoil gauss/coil type barrel, as as they carry the same electricity warning labels as UN weapons. In episode 7, Amos is seen readying a Martian assault rifle by inserting some form of cartridge into the stock of the weapon. As this is an unpractical location for a magazine, it is likely that this is a battery or power pack for its gauss/coil barrel.
While a gauss/coil gun would not be fully recoilless (thanks Newton…!) the lower power and lack of an explosion (i.e. no shockwave) would make this far more suited for shipboard operations.
The benefit of supporting two different firing modes is that it grants Martian forces far greater flexibility. Their rifles can fire low-recoil ammunition in situations where they absolutely do not want their bullets to damage the walls and doors of the ship or station they are on. However, the lower speed and mass of such ammunition also makes it less effective against body armor, as seen at the end of Season 1, Episode 10. By having a second barrel equipped for more powerful ammunition, Martian forces can rapidly switch from one to the other, allowing them to deliver devastating armor piercing and incendiary fire whenever needed without replacing their weapons.
Having a dual-mode weapon may however have forced the hand of the MCRN in the choice between a conventional and a bullpup design. Even in the lower gravities seen on Mars and elsewhere in the system, a double barrel design concentrates a lot of mass and weight in the forward part of the weapon. A bullpup design moves the weight and mass of the barrels closer to the shooter, presumably making the weapon more manageable than a much longer conventional rifle.
Real World Inspiration
Like all other small arms in The Expanse, the Martian Assault Rifle is also based on an airsoft gun, the APS UAR (Urban Assault Rifle.) But in contrast to the other airsoft weapons seen in The Expanse in season 1, this one is not based on a “real” gun, and was designed from the start as an airsoft.
Interestingly, the battery that powers the second version of the UAR is also placed in the end of the gun, exactly where Amos was seen inserting a cartridge in episode 7. The UAR, however, requires the gun to be opened to insert the battery, which means the designers must have made some adaptations to allow Amos to insert anything.
The gun is also seen briefly in this excellent interview Adam Savage did with the prop department of The Expanse back in 2017.
The Weapons of The Expanse 
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