Another vote for the Kifaru Gunbearer. Have used it for 10 years and put many, many miles on it. I modified it a bit (beefed it up at upper attachment point) to better handle a 12 lb. 338AX with 28" barrel, but I still think it's the best design out there for comfortably carrying a rifle while backpacking AND you have faster access to it than if the rifle was slung over your shoulder. It can be ordered separately and fit to many packs, but I'd reccommend ordering it with a Kifaru pack attached to it...
Tim in Washington, have any of you guys tried the new Timberline? Looks like a great hunting pack. I've got a Gen 1 frame so when the duplex Timberline bags just came out, I ordered the Duplex Timberline 2 that attaches right to my current frame. Am looking forward to to trying this out hiking this summer and hunting this fall.