Infinite Sword Ammo In Campaign:

This is quite simple. Find a sword and use all of its energy, (The Arbiter and Delta Halo are ideal), and complete the level. Watch the cutscene, but make sure the empty sword is out. In Delta Halo you learn of the Prophets intention to activate Halo, and in the Arbiter you just land with the Banshee. Afterwards, your sword will come out empty, BUT, if you attack an enemy, the sword will work as if it had energy. Note, this will not work if you complete a level and switch to another character, or if you start another level where you can't keep the sword. Also, i'm not sure about restarting levels or reverting to past checkpoints, so I don't recommend that.