Unlimited ammo

The Unlimited Ammunition glitch can be done in Halo 3 multiplayer and campaign.

Walkthrough

Campaign

One way to get unlimited ammo is to give an ally a weapon. Even if there is only one round or 1 unit of energy remaining when you give it to them, the weapon will never deplete unless you use it again, but you will still need to reload.

Another method is to play Co-op, and continually kill your partner in a safe zone to amass a huge amount of dropped weapons.

Multiplayer

Edit a level in custom games by pressing the X button on the controller on the main custom games screen. Then, you can set all weapons on the map to have unlimited ammo.

Some Halo 3 matchmaking types, such as Team Snipers or Griffball, include unlimited ammo. Most types, with a set weapons list, include unlimited ammo.

Forge

This glitch is mostly just having the right timing. Pick up a weapon in player mode ( Spartan Lasers, shotgun, and sniper rifles are the only weapons that has been tested and worked so far) then fire your weapon and go to moniter mode at nearly (only a fraction of a secind should be needed)same time. If worked propely you will hae same ammo lost then it will in about a second you should have ammo replinished as if you never fired. This WILL cause damage and it also has been noted to NOT WORK IN LOCAL GAMES.