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I am carrying a mid-weight rifle 12 lbs.
I want the rifle to be somewhat hands free for bino work but at the ready for any close encounters, so a cross chest carry would be needed.
I also have some crawling and climbing and for this a middle of the back would be the preferred location.
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Don't know if you're referencing 'tactical' due to tactical situations. However for hunting I use a very unconventional but very comfortable sling that meets each of your criteria.
I carry an almost 12# rifle also.
I carry across the front.
I can pretty much instantly be in shooting position with a natural move. I jokingly say that I can miss 2 shots before the other guys get their rifles off their shoulders. Jokingly but sometimes accurate [img]/ubbthreads/images/graemlins/blush.gif[/img] [img]/ubbthreads/images/graemlins/blush.gif[/img]
The sling snaps so that the rifle will not shift while climbing western PA spoil piles or Idaho mountain shale out crops.
It remains in place with no intervention while walking etc.
The only down side is that bouncing binos clunk on the fore arm if not careful [img]/ubbthreads/images/graemlins/frown.gif[/img]
Here's a pic.
That's masking tape on the muzzle. not snow....