Instructions
Pick up a rocket launcher. Typically, one or two rocket launchers are spread throughout a multiplayer match, so you will have to be fast to reach the weapon before anyone else does.
Pick up a human machine gun or a hive cannon by pressing and holding the "-" button. Either of these weapons will work, as they are automatic weapons.
Drop the weapons in front of a human ammo box. These yellow rectangular boxes are spread throughout a variety of single-player and multiplayer matches. You will need to be close enough to the ammo box to pick up the weapon but still receive ammo from the box.
Switch the rocket launcher to your second weapon slot. This will allow you to pick up a weapon without dropping the rocket launcher.
Pick up the weapon on the ground by holding the "-" button and press the switch guns button in the middle of switching, so that the gun is thrown on the ground. As it is being tossed on the ground, quickly point to the human ammo box and press the "A" button.
Pick up the rocket launcher, and you will be able to fire the weapon rapidly without ever running out of ammo.