Use your knife to collect more ammo for the other guns. By doing this, you can get full ammo for every gun you have. If you are using a flamethrower in the later levels, use a short burst to conserve fuel. Use the magnum when low on gas canisters for the flamethrower. The rocket launcher does serious damage. Use the primary fire button to fire a single shot. Shooting multiple times is a waste of rockets. You can use this gun on single armored foes or groups of enemies.