Browning has a sling in its catalog that employs two straps, one for each shoulder and the rifle goes up the center of your back. I have used them for longer carries and they work very well.
http://www.browning.com/products/cat...&offset=26
I prefer the super quality slings from
www.texashuntco.com or
www.specops.com, both the same outfit I believe. They are about the toughest slings I have found, have a piece of bungie cord in them so they stretch.
I had a bad experience with a stretchy type sling from Vero Vellini, very simply it was not strong enough for toting a 14 pound rifle and it came apart.
The Kifaru pack is a great setup, you can carry the rifle in a little holster and locking strap that works great but I believe it would be best with lighter rifles. Would be pretty awkward with all that weight on one side. I believe you are on the right track with your idea of slinging down the middle of your back.
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