This has to be done on co-op.

On the level Halo, on the bit when you find all the flood, if you look straight ahead you will see the inclining tower.

Go round and stand by the edge, stand on another player's head and grenade jump and stand on the ledge.

Walk up till it levels out and you will see to your left Sergeant Johnson with a Spartan Laser.

Melee him off and he dies, jump down there and pick it up (do it before he respwans!)

You should have infinite ammo for the laser.

[unconfirmed]