I built a "Kifaru Gun Bearer" type rig that attaches to my frame pack. Slick as snot and will even secure my 12.5" 870, though not much room to spare. I like the versatility of a frame pack for huinting because you will inevitably have to pack heavy loads and for that job the frame pack is the king of the hill. I can't see how any internal frame pack could carry the load of a properly set up frame pack though it would appear that people have carried some staggering loads in the big Eberlestock packs. But on the US forums the trend seems to be to take the Eberlestock out as a day pack, shoot an animal and briong the first load out int he Eberlestock then pick up the frame pack at the truck and use it to pack out the rest. Seems to me that if you're dumping $400-$500 on a pack you shouldn't need to pick up a second pack for $150 to complete your rig.
The other bonus to a frame pack is that you can lash things to the outside of the pack to save the internal space though some of the bags available have decent external pockets.
But backpacks will invariably end up being too small, no matter what you buy. I'm sure that there are guys out there with V90 Battleships that are #####ing that the pack is too small for their uses.