Instructions
 
Go to the top of the tower in Halo, where Sergeant Johnson is firing on the Flood from a cliff. Do not kill the Flood yet. 
Knock Sergeant Johnson off the cliff with a rocket launcher or grenade; he will drop his Spartan laser as he falls. 
Pick up the Spartan laser Johnson dropped. This is not the infinite-ammo version, but you must pick this version up to get the infinite-ammo version to spawn. 
Kill Sergeant Johnson, who will not stay dead. When he comes back to life, he will possess the infinite-ammo Spartan laser. 
Kill Sergeant Johnson again; he will drop his new laser. 
Pick up the dropped Spartan laser. Although it displays a charge of 100, the charge does not deplete when the weapon is fired, allowing the laser to be used indefinitely.