I just picked up a Vorn Equipment backpack in 20L. It has a quick release for the rifle and you don't need a sling. They're running a sale through the end of this month.
http://us.vornequipment.com/
I tried to carry my 300WM with a suppressor, and slung over the shoulder, it would be pretty unruly and I had to constantly readjust. With this pack (just tried out at home), the rifle secure and stable. It comes to the ready very quickly.
https://youtu.be/wXHJ_PuPSJc
