From Halopedia, the Halo wiki
The Asymmetric Recoilless Carbine-920 (ARC-920), also known simply as the railgun, is a compact-channel linear accelerator weapon used by the United Nations Space Command. It is designed and manufactured by Acheron Security.
The ARC-920 railgun is a result of scientific breakthroughs following the Human-Covenant War that allowed for portable, hand-held, electromagnetic launcher systems to be used in combat. The ARC-920's disposable power cell and ferric-shelled M645 FTP-HE projectile has dramatically decreased the weapon's total mass. Despite the ARC-920's lighter weight over previous models, it is still too heavy and cumbersome for general use by non-augmented personnel.
The ARC-920 uses a pair of parallel conducting rails to accelerate its projectiles. A large electric current flows from one rail into the projectile and then to the other rail; the electromagnetic effects of this interaction accelerate the projectile to an immense speed. The rifle requires a brief charging period before firing high-explosive rounds accelerated to an incredible speed. The explosive yield contained in the round adds to the kinetic force of the impact, making the weapon effective against both infantry and armor. The weapon lacks a scope. The ARC-920 is one of the various limited-production electromagnetic launchers in service with the UNSC Armed Forces, and is widely considered the best among them.
Compared to the Spartan Laser, the ARC-920's charging time is half the duration and deals less damage to vehicles. In addition, the traveling time for a fired shot is almost instantaneous, though still markedly slower than most ballistic, hard light, and laser weapons in the game, which are hitscan. In Campaign and Spartan Ops, the charging time is longer than in War Games.
The railgun must be charged before firing, which is a disadvantage on the battlefield. It can only fire one slug before reloading and is weak against vehicles. Its lack of scope coupled with its narrow blast radius make it difficult to aim, limiting mostly to firing at stationary or slow-moving targets.
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