Scarab Gun
From Halopedia, the Halo wiki
Summary
In the Halo 2 level Metropolis, there is a hidden Plasma Rifle with the power of a Scarab's main cannon. It has been dubbed the "Scarab Gun".
Directions
At the begining of the level Metropolis in Halo 2, kill all enemies on as you go over the bridge, and when you get to the end, destroy all the banshees except for one. Lure the one left over down the tunnel with a Ghost, shooting to keep its attention, until you reach the loading zone. The Banshee's pilot will disappear and you will have about a second to board the Banshee before it disappears. You should have the "Press X to board Banshee" message as you trigger the loading zone. If you board the Banshee before triggering the loading zone, it will explode and you will die instantly. You then must work the Banshee through the tunnel and out the other side. Fly to the left and you will see two bridges suspended between skyscrapers. You will find the Scarab Gun on the first bridge, hovering a few feet off the gound.
