M921 Asymmetric Recoilless Carbine

"Maybe not as precise, or as elegant as the M99CA1, but when it hits somebody, they're going to be washing them off every surface for a week."

- Anonymous UNSC serviceman

The is a UNSC railgun used for anti-structure/materiel/vehicle purposes, being capable of demolishing most targets with ease. While it lacks the high power of magnetic accelerator weapons, it possess a higher rate of fire than similar sized platforms, and with specialised munitions, it can be used to take down a variety of different targets.

Uses
The M921 ARC is mainly used by weapon specialists or special forces, when heavy weapons such as the M41B2 Medium Anti-Vehicle/Assault Weapon or M66 Light Multipurpose Assault Weapon are too large, too imprecise or cause too much collateral damage. Being highly accurate, with dialable power settings and a wide variety of munitions for as many situations. Its often used as a substitute anti-armour weapon in urban situations, or special operations that are close to civilians or delicate material that warrant a more controlled weapon. It can also be used as a heavy sniper weapon, capable of eliminating targets without fear of survival. It can use armour piercing, fragmenting, multiple flechette and explosive rounds for a variety of purposes.

It is most commonly deployed by Special Forces weapon specialists, though sees occasional use with UNSC Army and Marine personnel.

Operation
The weapon functions by using two magnetics with opposite charges to propel a ferro or paramagnetic slug to high speeds. Due to the nature of its propulsion, it can be exceedingly accurate out to long ranges. This is feed by a 5 round box magazine and a feeding mechanism that places the slug between the two rails, ready to fire. The rails are produced from high heat resistant material, allowing it to sustain fire for long periods before it begins to undergo thermal distortion, beginning to affect accuracy. The rest of the components are high heat resistant polymers, and lightweight alloys optimized and engineered for maximum heat resistance.

Both rails have Doppler Cooling banks that are made up of an array of tiny LASERs that are arranged in opposite directions. As excited atoms and molecules enter the cooling bank, they gain photons from the laser they are moving away from, via the doppler effect. These atoms then slow to a crawl, due to the cancelling effects of the gained photons and are subsequently chilled. As the barrel continues to heat up, due to thermodynamics, the heat will move from a place of high temperature to a place of low temperature, constantly cooling the weapon until equilibrium is made. As the weapon cools down, the opposite becomes true, with the molecules within the bank moving to warm up the weapon as it drifts below equilibrium. This keeps it closer within optimum parameters but are energy prohibitive.

The gun is powered by a hydrogen power pack, in the butt of the gun. This power back is good for 30 shots, and must be replaced as the gun will cease to fire or cool. These packs are small and lightweight, and are non-explosive and non-heat reactive.

Other inbuilt features of the gun include non-electrical controls such as the magazine release and safety, and an advanced fire control system, allowing the power level, and consequently distance to be varied. The fire control system shows maximum range, and predicted penetration of light and hardened materials. The FCS also carries two hardlink ports. Along the upper receiver is a Misriah rail, allowing a scope to be mounted and linked to the FCS, increasing the accuracy of the FCS predictions. The lower receiver also carries a accessory rail, allowing for the mounting of grips or bipods.