Gravity rifle

The Gravity Rifle was going to be the Covenant Sniper Rifle of Halo: Combat Evolved, but Bungie decided to delete this weapon much like they deleted the Engineer. They do have the model in game but they never made a proper texture, first person model, and projectile. However a common mod simply has it firing plasma grenades which seems to be the right idea for a supposedly powerful weapon, similar to a Fuel Rod Gun. The Gravity Rifle was remade for Halo 2 as the Beam Rifle, which was also in Halo 3.

Halo: CE
In Halo: Custom Edition, there is a cheat which you can use to get it. If you activate devmode ( -console -devmode), exactly as it's typed here with spaces, press ~ without the shift, and then type "cheat_all_weapons" into the purple text space, on the bottom of the screen, and then press enter and ~ again. It is situated next to the Flamethrower. To pick it up, the screen will say "Press E To pick up ", as if the game designers didn't even give it a name. If you press E and pick it up, you can't see it unless you change the camera position (` debug_camera_save (press enter) debug_camera_load (press enter) `). There is no animation for it being fired, but it will start a fire. It's the exact same as a flamethrower, except it only starts a fire when you point it at a solid object, like the ground, a wall or a vehicle. Also, it has unlimited ammunition, so it will never run out. If you get a user-created map, some have replaced the gravity rifle with other weapons, or may not, have it at all.

Images
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