Scarab Gun

In the Halo 2 level Metropolis, there is a hidden Type-25 Directed Energy Weapon (Plasma Rifle) that looks like the normal Plasma Rifle with the power of a Scarab's main cannon. It has unlimited ammo, continuous rapid fire, and it never overheats. However, if you fire this gun too close to your position, you will die instantly due to the immense splash-damage. It is dual-wieldable, just like a normal Plasma Rifle (but there is no real need to dual wield, the gun is more powerful than any other weapon or combo in the game by itself, and wielding it along with any other weapon will not help much even if you dual wield two Scarab Guns). The Halo 2 Scarab Gun is the most powerful weapon of the entire Halo series, causing more destruction than the Halo 3 Spartan Laser and even the Scarab cannon in Halo 3. Even on Easy, with your high shields, it takes a great of amount of skill to not kill yourself.

Method 1
Note: The Sputnik Skull is recommended for this, but not necessarily required. At the beginning of the level, Metropolis (Easy difficulty recommended), kill all of your allies that have Rocket Launchers and take them. It is not necessary to kill the enemies on the bridge. Simply driving past them, is sufficient to obtain the Scarab gun, but if you are playing on a higher difficulty, you may have to do so. At the point where the two extra allies show up on the bridge, the first two Banshees show up. Saving as many Banshees as possible will help later so that should one be lost others are available. When the end of the tunnel is reached, destroy all the enemies but do not blow up any of the Banshees. Lure each Banshee, close enough to board it. Kill each enemy after forcing them out. Next, clear the tunnel ahead. Stop, once you reach the closed section of the roadway and have to detour off into the pipes. (Make sure to kill the sniper in there before heading back.) This will leave the tunnel enemy free, making it easier to work a Banshee down the tunnel to the spot needed. This is where the Sputnik Skull comes in handy.

Because your melees make objects move further, it will be easier to maneuver the Banshee where you need it. Return to the end of the bridge and move a tank to road on the bridge. Land a Banshee on top of the tank and then drive the tank down the tunnel. When you reach the first road block, you will need to position the Banshee on the top of the ramp to the right. It is important that you don't attempt to fly the Banshee once in the tunnel. It will cause you to die. You are not able to fly the Banshee into the tunnel due to an invisible wall. Now by hitting the Banshee you can maneuver it through the walk way at the top of the ramp and past the road block. To get the Banshee through the walk ways and end tunnel it may be necessary to shoot off the wings. To do this use either a Plasma Rifle or a Sniper Rifle and shoot single shots until the wings are deformed. It is important that you carry two different sized weapons (for instance, an SMG and a Rocket Launcher) because sometimes a larger weapon is too much and sometimes a smaller weapon just isn't enough. Don't hit the Banshee in succession, too many times. Allow it to rest between attacks, or it will blow it up. Two melees, is the most that the Banshees can usually take without needing to rest.

Work the Banshee past the second road block, by forcing it up the wall to the right and through the walk way again. Force the Banshee into the tunnel on the right and up the first ramp in the tunnel. You need to position the Banshee (once on the platform at the top of the ramp) so that it is facing into the next tunnel. Your Banshee should be upright and you should be able to walk up to it and see the "Hold X to pilot Banshee" message. Now force the Banshee, into the tunnel by using well placed and strategic hits. Once the "Loading ..." message appears you need to enter the Banshee before it disappears, If you don't make it you will have to revert to last check point and clear the tunnel again. If the Banshee is destroyed, it is necessary to go back and retrieve another and force it down the tunnel the same way.

After "Loading...done" disappears, fly through the tunnel and then directly up and to the left. There will be two suspension bridges; head to the first one. On the middle of the suspension bridge is a cone over which the Scarab Gun is hovering. Yes, it's as hard as it sounds.

Method 2
Requirements: Lots of patience.

First, kill all allies and eliminate everything but one Banshee and one Wraith at the section before the tunnel. Before taking the wraith, use the scorpion turret to shoot off the wings of the last Banshee. Do not shoot too many times in a row or it will explode.Steal the Wraith, luring the Banshee through the tunnel. The reason why you need a Wraith is to better protect yourself from the incoming super heated plasma bombs from the Banshee (the Banshee WILL shoot at you) and to easily kill your allies in a Warthog later in the tunnel. While heading down the tunnel, do not race too far ahead, as a checkpoint will be triggered and the Banshee will disappear. On a similar note, do not let the Banshee draw too close before getting to the lower part of the tunnel, because it will turn around and leave. Try to keep the Banshee within track of your radar. Kill all enemies in your way as you continue. Due to the AI wanting to do complex maneuvers, such as turning around or turning in general, it will occasionally get stuck in a crack. To fix this simply go directly behind it and move backwards until it turns around and gets itself out of the crack. Or, if it gets stuck near your enemies, you might want to leave it for awhile and kill the enemies first, thereafter returning to free it. This will ensure that it doesn't fly away while you're not keeping a keen eye on it. However, while doing this, you must make sure there is not too great a distance between you and the Banshee or it will disappear.Eventually when you and it are in the service tunnel, lure it to the point where the loading screen appears. You have to board it within 1 second. (Yes, I repeat, 1 Second.) Now fly directly up to a suspension bridge found in between two building (note that there are two bridges, go to the first one). Go to the middle of it and claim the gun.

Method 3
Requirements: Sputnik Skull, Black Eye Skull and an Overshield (Using Black Eye Skull).

First of all get the Black Eye Skull to get your shield to overshield range. Then drive a Warthog to the command outpost, and up the small bridge at the farthest from the outpost. Be sure your Rocket Launcher is either your primary or secondary weapon. Stand on top of the gunner part. Fire the rocket at the bridge. This is when the Sputnik Skull comes in handy, However, there is a high probability of death. Once on the bridge walk to the Scarab Gun and take it. Try it on multiplayer and hop off the bridge, die, respawn with another player and get your gun back.

Method 4
Requirements: Sputnik Skull and another player (strongly Recommended)

First of all, get a Rocket Launcher and a Battle Rifle. Then, continue until you get to the part where you have to destroy the Scarab (The last part of the level) and get as much ammo for your Battle Rifle and Rocket Launcher but not necessarily required. Then, zoom in your Battle Rifle at the last bridge where there is no shelter and the connector to the second building. Shoot the box beside the middle exploding vent. Once you shoot it 3 times, zoom out and head to it. Push the box and align it with the drain at the start of the bridge. Make it as perfect as possible, then jump on top of it. I strongly recommend you wait for a checkpoint so you don't have to repeat the entire procedure all over again. Once the 'checkpoint...done' appears, switch to your Rocket Launcher and shoot a rocket near the vent nearest to you. You will then fly up towards the bridge where the Scarab Gun is located. This takes some tries. Sometimes, you will go too low, sometimes, you will hit the bottom of the bridge and mostly, you might go too high and hit the invisible ceiling and fall to your death. But after a few tries, you will be able to get it. This is far most the easiest way to retrieve it. I suggest you do in Co-Operative mode because when you retrieve the deadly weapon, you will not be able to get down. In co-operative, you can position the second player directly under the suspension bridge. Then, drop to your death and the second player would most likely retrieve it.

Other Scarab Guns
There are actually 4 Scarab guns in Halo 2. Two on Outskirts and two on Metropolis. This one was originally reported by dantheman87: On Outskirts with the black eye and sputnik skulls on get a Rocket Launcher and grenades. Get in a Ghost and go to the highway tunnel. If you go against the wall and out toward the water you can slip outside the map. Go to the edge of the building and turn left. Now go boost forward until you get a checkpoint. Then when you do get out and aim at the ground. Throw a grenade and shoot the ground with a rocket and at the same time jump. With an overshield you should fly up in the air. Hold X and you should (if in the right place) pick it up and land on the wall. You now have another scarab gun. For the next one you can only get to it with modding. When the scarab blasts a hole in the tunnel in Outskirts, if you can get outside the tunnel before the scarab stops firing, it is in a blue cloud beneath the scarab. The next one is in Metropolis. On multiplayer, fight until you clear the city center and talk to Corporal Perez. Have one player stand where the scarab arrives. Have the other go into the building and load the scarab. When it loads have the other player jump, with either the overshield, rocket, grenade, sputnik jump or another into the scarab's laser or if there's a blue cloud into that and hold X and they will pick up yet another scarab gun.

Reasons for Existence

 * The Scarab Gun is rumored to exist because of a programming shortcut used by Bungie. The Scarab's weapon actually is the Scarab gun. Bungie essentially made the Scarab "wield" the Scarab Gun, rather than program a weapon attached to the Scarab itself.
 * Some speculated that Bungie could have put it there for players messing around and trying to get out of the map to find Skulls and other Easter Eggs.

Trivia

 * there were rumors that there is a scarab gun in halo ce but unlikely cause there is no scarabs shown in the game anyways.
 * The Scarab Gun has a slightly different appearance than a normal Plasma Rifle. On co-op, when you get the gun, have your partner look at you while you are dual-wielding it with a Plasma Rifle. There is a glow at the tip of the Plasma Rifle, that is not present on the Scarab Gun (although you can do it on single player).
 * Unlike the Plasma rifle which fires blue bolts, the Scarab Gun shoots out huge green bolts which effectively obliterate anything in your path.
 * An achievement in Halo 2 Vista called King of the Scarab is unlocked by obtaining this weapon.
 * Interestingly, the safety cone has multiple "Danger" signs on it, showing that the gun might have been put there intentionally.
 * There is another Scarab Gun in Halo 2. It is found on the level before the other one can be found, Outskirts. The Scarab Gun can be found underneath the moving Scarab and is able to be picked up after doing an explosive jump with the Sputnik Skull, the Black Eye skull is also necessary to survive the explosion. The video tutorial can be found on Youtube, it's titled "The second scarab gun".There are 4 Scarab Guns in halo 2. Two On Outskirts and two on Metropolis. However, one on Outskirts is unattainable without mods.
 * There is no Scarab Gun, in the level Great Journey in Halo 2. Though there was rumored to be one, modders who looked at the map file for The Great Journey proved that no such gun existed in that level, even though the scarab fires multiple times.
 * If the player gets into the side seat of a Warthog and fires the Scarab gun, the screen (Except for the HUD) will go completely white until the player stops firing. That's why, in third person, the weapon's gleam is as large as the Scarab's main gun.
 * If you give a marine this Easter egg, he will hold it as if it were a normal plasma rifle, but he will never fire it. There is a lag, however, before the weapon appears, hinting that Bungie might have expected this and took steps to prevent it.
 * The reason the Scarab gun is floating above the traffic cone is, presumably a Weapon Holder is placed inside the Traffic cone.