The military doesn't generally teach this type of stance due to all the stuff we keep on the rifles. 3gunners don't run lights, laser, etc on the gun. If they do they are generally much smaller instead of being super robust, and therefore larger. Not only that, holding the rifle the way 3gunners do for hours at a time would cause undue fatigue. 3gunners carry the rifle that way for a few minutes at a time, maybe a hour or so at a time if they are practicing. Military carries the rifle for hours on end, days on end. There is also the tunnel vision aspect of it. For some people, it blocks your vision to the weak side. Puts my shoulder so high I can't see anything to my left side when I tried it. In 3gun, no one is shooting back.
Is it better than the way the military teaches? That's up for debate. Rifle is one of my strongest suits in 3gun due to the ammount of shooting I did with it in the Marines. The guys that beat me on rifle do so because of practice and skill, not the way they hold it. If they had trained to hold it another way with the repitition they do with the 3gunner style, they would be just as good with it.
Try it the way they do it, and if you like it go with it. I prefer a modified version of it, were I use a vert grip as a hand stop. My ring and pinky finger will be on the grip, other 2 fingers on the bottom of the handguard, and my thumb will be in the side of the handguard instead of a finger. It's the way I was taught, and the way I am comfertable doing it. There are tons of ways to hold a rifle, each has it's benifits and down sides. Figure out when works for you, and stick with it.
