The Gravity Rifle was going to be the Covenant Sniper of Halo: Combat Evolved. 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.
Template:Deletemat (Indeed the tag is customized for Halo 2's Deleted material, but you get the point.)
Halo: CE
In Halo: Custom Edition, there is a cheat which you can use to get it. Type the button next to the number 1 (`) and then type cheat_all_weapons 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 <need string entry here> 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 possition (` debug_camera_save (press enter) debug_camera_load (press enter) `). There is no animation for it being fired, but when the the ammo hits the ground, it will start a fire.
Images
- GravityRifle.jpg
Developer Mode Screenshot from Halo PC in Battle Creek