Unlimited Energy Sword
From Halopedia, the Halo wiki
Summary
Go onto a two part level. Then on the first part, grab a sword and use it until it is empty. Instead of switching it out, keep it and use another weapon. Then, after the second part of the level starts, you will still have the handle. Even though you get the message saying "out of ammo", the game will mistake it for a sword and this glitched sword will have unlimited ammo.
Also if your sword runs out of ammo in normal playing, you will get the brass knuckle. It is just a very fast meleeing weapon that does the same damage as a melee from a plasma pistol.
Another way is to simply hit the melee button instead of the "fire" button.
