The first Expanse-weapon seen and used in the show is the UN, or UNN (United Nations Navy) Assault Rifle. First seen when carried by a UN sentry outside the Black Site visited by Avasarala i Episode 1, it appears to be in widespread use throughout the system by UN and UN-related elements, including certain members of private security organizations (such as Amos Burton, an erstwhile employee of the Pur & Kleen-corporation, and the security forces on Tycho station.) Where many other weapons on the show are in a more compact bullpup-configuration, where the grip and trigger are in front of the magazine and firing mechanism, the UN rifle is among few long arms in the show that do not. As a result, the rifle appears to be longer and more cumbersome in use in confined spaces. Curiously, despite being this widespread, it is also one of the weapons we very rarely see actually being fired!
Ammunition type or capacity is never specified, but like the Star Helix Pistol the rifle is also fitted with an electricity warning. This may indicate that this is also potentially a gauss/coil gun, which uses electricity to power the projectile forward, despite the clearly visible muzzle flash seen when the gun is fired.
Real World Inspiration
The UN Assault Rifle is a modified version of the SL8-4, which itself is a civilian version of the G-36 Assault Rifle made by the German company Heckler & Koch. The military version of the rifle is in use by several armies around the world, including those of Germany and Spain, and has seen extensive use movies and TV-shows due to its futuristic design.
Modifications on the show include a fixed sight or potentially laser pointer mounted on the upper receiver, and a thicker hand grip under the barrel. Under the muzzle we see a square with a riveted surface. Its purpose is unknown, and has likely been added to bulk up the design, but it could be a sensor of some sort to support in aiming.
Gallery
The Weapons of The Expanse 
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