The Infinite Ammo Glitch is a glitch in Halo: Combat Evolved (on both Xbox & PC versions) in which any weapon that requires reloading can obtain infinite ammo. However, the glitch will have to be done each time a reload is required. To do this glitch, take any weapon (except for the Plasma Rifle and Plasma Pistol ) and empty the magazine. When the magazine is empty, before the reload animation begins, switch the weapon with any other weapon on the ground. The game will think that the weapon is still being held & that it's being reloaded but when the weapon is picked up again it will have the same amount of remaining ammo as when it was dropped.
There are a few tricky weapons that need to be watched out for. They are the Pistol, the Sniper Rifle, and the Shotgun. The Pistol and Sniper Rifle sometimes won't begin the reloading animation & the glitch will have to be re-done. They'll also, every so often, actually reload and therefore use actual ammo. With the Shotgun, seeing as each shell is individually loaded, it's best to shoot all but one shell, stand over the weapon on the ground, shoot the last shot & switch to the secondary weapon, quickly switch back to the Shotgun & then switch with the weapon on the ground.
All weapons that the glitch are possible with will still make their respective reloading sounds. For some weapons, it's possible to physically see when the weapon has reloaded. They are the Assault Rifle, Needler, and Shotgun. With a scoped weapon, it's possible to look on the Assault Rifle's little screen & see the magazine ammo count go from "00" to "60". With the Needler, more needles will appear on the top of the weapon, & the Shotgun will self-pump itself. In Co-op, if player A performs the glitch correctly, player B can watch player A's character do the reload animation even though they're not holding the weapon being "reloaded".
In every other Halo Game (except Halo Wars) the glitch is performed by switching weapons with an ally NPC when the player character's weapon's magazine is empty. It was speculated that this glitch would be removed in Halo: Anniversary; however, this turned out not to be the case.