For those born without a sense of humour, Halopedia has an article on ARC-920.
Cquote1Why isn't this thing working?Cquote2
— A Marine firing the Railgun at a vehicle.

The Railgun, if charged, uses magnets to fire a blast of Gruntiness. In doing so, it can should be able to kill most anything, but because it was manufactured by puny earthlings, it is cannot destroy much

It can't even kill a Mongoose, but is it good at destroying Spartans, if you aim for the face. A common theory is that 343 Industries wanted to see how useless of a weapon they could develop, and then implement it into Halo 4.

Melee Weapons: Kitchen Knife | Japanese Butter Knife | Taser Stick

Handguns: Comfortable Pistol | Another Comfortable Pistol | WTF no scope? | M6C/Suck em' | God's Sidearm | Safety Mode On | Trusty Sidearm | Trusty Sidearm 2: Gradius | Assassination Pistol | Carbine Pistol | Silenced Pistol | Orbital Airstrike

Automatic & Semi-Automatic Weapons: Bullet Hose | Silent Bullet Spitter | Insult to Rifles | Revised Insult to Rifles | Chronologically Confusing insult to rifles | The insult to rifles that actually kicks ass | Bee-Arr | Bullet Spammer | Another insult to rifles | Large Bullet Hose

Other: HEADSHOT! | Pest Control Tool | Mini Missile Silo | Rocket Lawn chair | Splazer | Campergun | Noob Obliterator | Crude Thumper rip-off | Supersonic Rail Exploder | Exploding Cake Detonator | Kill The Hydra

Grenades: Damn, no stickies | Bang Grenade | Ear Bleeder | "I embrace y'all with napalm..."

Melee Weapons: Energy Shank | Hot Poker | Death Stick | Energy Shiv | Gravy Hammer | Monkey Fist | Monkey Shank

Other: Peashooter | Overheated | Angry Plasma Rifle | Overheater | Noobler | Larger Noobler | Long range Noobler | Nailgun | New Noob Combo | Rock Slinger | L337 5K1LL5 | Follow the pink light | Vacuum Quadlazer | Jelly Launcher | Semtex Blue Spider | Stick-rock | Fire in the Box

Heavy Weapons: Mini Blue Jelly Factory | Death impersonated | Godly Plasma Rifle | Huge Water Gun on Spaceships | Hax Gun | Smaller Hax Gun | Derp Gun | Magic Grenade Thrower

The Quadlazer | Zapper | Tracer Rifle | Yellow Light Spammer | Campergun's Retarded Brother | Triforce Gun | Promethean Glory | Forcefield | Bee Grenade