The goal is to achieve as much Bone Support and Muscle Relaxation as possible. The further you get away from the ground and the more mobility you add to the shooting, the less MR and BS you can apply. This does not mean that you don't attempt to achieve them. The more of these two fundamentals that you can apply the longer you can shoot well. It will also make it easier to shoot accurately and longer.
What are they and what do they mean?
Muscle Relaxation is just that, do as little work as you can to achieve the same result of keeping, sight alignment and sight picture, while doing as little as possible. There are two parts of this. The stronger your system is and the better your trunk can fire, read the "CORE POST"
the less that your appendages have to work to maintain proper fundamentals. The next part is be as lazy as possible. By being strong and lazy you will have better marksmanship endurance.
If you have a good NPA (natural point of aim) then you have good muscle relaxation and bone support.
Well then why break it down into 3 parts?
Many people think they have achieved NPA but as soon as they think of the other two they realize that they were still too tight and/or pulling the weapon onto the target. It also helps to remember that even when kicking in doors, find these 2 as much as possible.
Bone Support is the action of trying to get the skeletal system to do as much of holding the weapon as possible. This is obviously more important in precision long range shooting but think of this a tad more deeply. If I am wearing body armor, ammo, grenades, radios, (kit) on my torso and having to shoot, how much more important do you think it is to have the skeletal system in proper alignment so it can handle more load. Hold a sledge hammer out in front of you but keep your hand close to the head. Now, do it again but hold the very bottom of the handle. It is exceptionally harder. The stronger your trunk the better your shooting "posture" the easier it is to shoot well for an extended period of time.
Use the sledge hammer analogy again but this time attach the sledge to your back. Oh wow it is so much easier to carry around. Use only the amount of strength necessary to accomplish the goal of good, fast fundamentals for accurate shot placement.
Also think of throwing a punch. If you are tight you have to let go to throw the punch quickly let alone hard. If you are too tight you will not be fast enough and you will get tired.
In short chill out and stay relaxed and get fit. Be able to pick up heavy shit quickly and your shooting will get much better.