Oh, and here's another (older one). It was right after my basic training (private second class), and I'm manning a german Heckler & Koch G36 5.56mm rifle :
The position could use some slight improvements (right elbow more against the chest, left foot toes extended instead of "folded", overall position less "graspy"), but you can see the fundamentals :
1- three-points triangular ground base : 2 feet and one knee. Way more stable than standing !
2- use your knee as an elbow rest, with your lower arm (NOT the elbow itself !!!) resting against your kneecap.
3- right elbow/arm not horizontal (called "chicken leg", as you can see in bad movies)
4- body slightly leaning forward.