Article: You need to practice lying on the ground. You will find that lining up your sights will be quite different and rapid fire become difficult to control. Your stance on the ground should be such: lay on your belly. Then roll slightly to the firing side. Place your support knee and elbow down on the ground. You will be a bit sideways, but your firing arm will be completely flush with the ground with your head resting on it looking down the sights. This allows burden-free breathing and a very stable platform. This can give you a very stable platform for shooting accurately. Put your firing leg behind you and essentially sit down on the heel of your foot. Your firing knee will be on the ground and your toe. Rest your support elbow on your support knee. You are in a tripod position (support foot, firing toe and firing knee), and you are in an arrangement where you can move quickly (practice moving periodically to and from the crouching position). Hiding behind cover in a gun fight seems like a smart idea, but don't think you are invincible behind most materials. There is a big difference between "cover" and "concealment". Objects like car doors and walls may only serve to keep you hidden from your opponents' view (concealment), but in most cases bullets can pierce through these objects. It is more advisable to look for something that can stop bullets as well as hide your whereabouts (cover). Shooting from behind cover should be done while exposing as little of your body as possible. If the cover allows it, do not expose your body from the same side.

What is a summary?
Prone: Crouching: Cover and Concealment: