The Spartan Laser Glitch can be performed when climbing your way up to the top of Control Room on the last level of Halo 3, Halo. As your climbing up to the top, you will see Sergeant Major Avery Johnson on the far left mountain side, helping you with his M6 Spartan Laser.
Also when you get up to the final level in Halo 3, battle 343 guilty spark as normal and then after Johnson uses his laser, get to him before he dies, swap any weapon for his laser (it will be on 69 although this does not matter) but you will not keep it after the battle.
How to
The trick is to shoot him until he falls off the mountain. Two minutes later, he will respawn near you. (Make sure you get him to fall off the mountain or will attempt to kill you, and usually succeeds).
When he is down next to you, kill him again (well, down him) and take his Spartan Laser, it will have either 3 energy left or 100, kill him until you get 100. That will have an unlimited supply, make sure you get two and leave it behind, otherwise when you kill 343 Guilty Spark, you will lose it.
METHOD 2:
wait until you open the door and prepare for the final battle. Johnson appears inside the hallway behind the door. Wait until you get Gravemind's message. THIS IS IMPERATIVE! If you kill Johnson before this, the Laser will NOT have Infinite Ammo. after you kill Johnson, take his laser. If you leave another weapon behind, he will take that instead of a new laser. I f in Co-op, have the 2nd Player take the ammo. Kill Johnson MULTIPLE TIMES to get multiple Lasers.
On Legendary, it is observed that if you kill Johnson multiple times, leaving multiple lasers on the ground, after the 343Guilty Spark battle, Johnson will still be alive with you, and his old Lasers will be on the ground, still with infinite ammo. It is also noted that Johnson can get infected by Flood. If he does not, he will disappear at a Loading...done screen.