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 Custom Edition
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 are swapping it for another weapon, it will say, "Press E to pick up ". 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 create a bright flash when shot. It's similar to the flamethrower, except it only starts a fire when you point it at a solid object, like the ground, a wall or a vehicle. The Gravity Rifle when shot, has no shot animation, but has a small flamethrower burst when hits. It is pinpoint accurate, despite having no reticule or scope. Some find it easier to aim by lining up a shot through a sniper scope, then switching and firing with the Gravity Rifle. 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 and some don't even include it at all.

You don't see the weapon since there are no FP animations for it, and the projectile is that of a flamethrower since Bungie did not design a projectile for it, or that Bungie never put it in.