Although the ARC-920 lacks a scope of its own, one can zoom in using the armor's built-in 5× zoom function, where the weapon can be charged and fired, with the zoom only being broken by the shot firing. This can be used to give greater precision to long-range shots. This tactic is only effective against stationary, very large, or slow-moving targets, however.
Against more nimble targets, the ARC-920 is best used at close to medium ranges, where its non-line-of-sight charge-up and significant aim-assist make it quite deadly. The long reload between shots, however, can make it a punishing weapon to use if the first shot misses, since the user will be defenseless for a fairly long before another railgun shot is available. In this case, it is advisable to retreat or switch to another weapon.
The weapon has a very small splash radius and a very high projectile speed, so unlike most other explosive weapons it is best used by aiming directly at the target's center of mass as opposed to firing at the ground below them in order to kill via splash.
If the user has any type of grenade, a fully charged railgun's firing can be delayed by around 1 second if said user throws a grenade. This is more efficient than switching the railgun out, which restarts the charging sequence. However, target lock must be re- acquired (if lost) in the 1 second delay.
The railgun is primarily an anti-heavy-infantry weapon, capable of killing Spartans, Elites and, on any difficulty below Legendary, even Promethean Knights in a single hit, but it also has some limited anti-materiel capability - it can kill Ghosts in two hits, and Banshees or Warthogs in approximately three. Against heavily armored vehicles like Scorpions, Wraiths, and Mantises, it is not very effective, unless the vehicle in question has already been severely damaged by other means.
In the campaign, the ARC-920 is only found in the missions Infinity and Shutdown.
In Spartan Ops, the ARC-920 features an additional advantage in the fact that, unlike the Spartan Laser, Rocket Launcher, and Sniper Rifle, a player can pick up additional ammunition from all UNSC ammo crates, giving the player a potentially unlimited supply of ammunition.
As with all weapons, the ARC-920 can now be "smart-scoped", eliminating one of the few issues with the weapon (its lack of a zoom function). This small increase in magnification can mean the difference between hitting a target and missing completely; use it liberally and without hesitation.
Besides that, its role and effectiveness has not changed very much. A single shot is enough to kill any Spartan-sized target (and do impressive damage to anything larger, like a Hunter) and it does impressive damage to vehicles. Use this to your advantage in Warzone, as its REQ requirement is much lower than that of a Spartan Laser.
Unlike in Halo 4, Halo 5: Guardians features the ARC-920 more commonly throughout the campaign. The ARC-920 is first found normally in Blue Team, but can also be obtained earlier via a Legendary-exclusive secret in Osiris.
Warzone also features two variants that split the weapon down its two lines of use; Whiplash features a faster charge time, two rounds per magazine, and airburst ammunition for anti-infantry work, and Arclight features a hard-hitting anti-armor round that can be held in ready to fire state indefinitely and does ludicrous damage to vehicles.
Recent nerfs and changes to the Railgun have made its overall use slightly less aggressive than before; the longer charge up time required means more planning is needed for a shot before it can be taken, but the amount of time that the shot can be held helps. Plan your target, then take the shot.