In the bunker level, if you go into the big room and look up, you will see monitors on the ceiling. Plant some remote mines on them and blow them up. Now you can fire rockets, grenades, tank ammo, or anything that is not a bullet and it will suspend in the air. It works best if you move backwards as you fire. The ammo is live so you can pick it up again or shoot it with a gun and it will explode. You can suspend the ammo anywhere in the level. It can be done in both single player and multi player modes.