Instructions
Pick up a bazooka, known as SMAW, in the game. There is typically only one bazooka per multi-player level, so you will have to move fast if you want to beat the other players to the location of the weapon.
Pick up a hive cannon or a human machine gun. Both weapons are common throughout multi-player levels and fire bullets rapidly.
Drop the automatic weapon you choose, either a human machine gun or hive cannon, in front of a human ammo box. The weapon will need to be close enough to the ammo box to pick up the ammo and still retrieve ammo from the ammo box.
Switch the SMAW to your second weapon slot. This will prevent you from dropping the weapon when you initially pick up an item.
Pick up the human machine gun or hive cannon that is near the ammo box. As you do, press the switch weapon button to drop the SMAW and also press the "A" button on the human ammo box pile.
Pick up the bazooka from the ground. If done correctly, you will have unlimited ammo for the rocket and be able to fire it continuously.