Well, it's more of a glitch. Infinite Spartan Laser - Play the last level. When that guy with the Spartan Laser comes down and opens the door, kill him. Take his S.P. and kill him again. Make sure not to leave any weapons lying next to him or let him grab a weapon with ammo. When he gets up again he should magically have another S.P. Kill him and take his S.P. It should have infinite ammo. Then run away before he kills you. This is useful for the last boss since it's tough. Works for 2 players.