To get infinite Spartan laser in the "Halo" mission, shoot Seargeant Johnson until he falls down and drops the Spartan Laser. Pick it up and continue to shoot him until ke kills you. When you respawn, shoot him until he falls down again and pick up the laser. He will get back up with a new laser on his back. Shoot him again and pick up the new gun he drops. It will have unlimited Spartan ammunition.