Re: prone with a sling
I would only add that I prefer a lower position, left elbow under the rifle. Not exactly a position you can get into without some practice, but when and if you can, extremely stable. I suspect Charles uses something similar.
And, shooting off a sling, one is not likely to be straight behind the rifle as you might be shooting off a bipod or rest, but instead, you are off at an angle to the target, say 30 degrees or so.....not a firm number, but an estimate, as your NPA is going to dictate how you are oriented to the target. Position is much more critical when shooting off the sling.
John