Quoted:
WAIT!
I thought having your elbow sticking out like that was proper stance, so tucking your elbows in standing is normal? I just got ragged by Sniper_Wolfe for not putting my elbows out, it does seem like the rifle is more secure when your elbow is out though.
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Actually, the old Army taught you to hold your elbow level with the ground just like TMAN is doing. (He learned that with Custer just before he was discharged and Custer went to Little Big Horn.)
Nowadays, probably due to clearing rooms and going through doorways, you are taught to keep it down next to your side. In fact, SWAT teams are taught to face the target squarely and keep both elbows tucked in to make maximum use of body armor and to diminish the size of the profile.
At Thunder Ranch, in the Urban Rifle course, Clint teaches you to keep the elbows in tight.