Kuiu has two options the quick disconnect if you are a hunter and need to draw quickly from the pack. I originally picked this, and it has more limited options for mounting. I switched for the standard setup. Both have the ability to carry on the side or the middle of the back of the pack. They both have the ability to adjust the stock vertically for longer rifles and tighten it up for shorter rifles. The adjustments give you the ability to snug it up to the pack and not have it move around, but still get to it quickly. The pack I have is not a framed pack, it is a day pack with an internal light frame, and I don't hunt so don't have a comment on extended carry. My RPR fits, and with the pack loaded is suitable to carry for my needs, to and from shooting positions where the hard case is not suitable.
p.s. They have some good videos on the quick release and mounting if you are interested, goodle Kuiu rifle holder and the vids will show up.