Haven't seen that before. Pretty neat.
Personally, I rarely carry a pistol unless in Grizzly country so I prefer a chest harness so that it is always on me, but I could see the benefit of this for general hiking/hunting. It would obviously need to be very securely attached as it could be ripped off putting the pack on and off. I've had issues with plastic parts breaking on some holsters in that price range in the past, so I'd look into that closely.
Assuming that it is indeed secure, then I can only see a couple really minor downsides. One being it will add some weight to that one shoulder strap.
The second is it might be difficult to sling your rifle over the same shoulder as the holster, but I switched from a sling to a gun bearer anyway.
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