They aren't necessarily cheap, or especially light, but they are built like a tank. I have the Gunslinger II. It has a dedicated compartment for your rifle that allows it to be removed and shouldered without having to remove the pack from your back. If you want a serious hunting pack that can carry a load, stand up to some abuse, and allow easy access to your rifle, they're worth a serious look.
Most standard backpacks are not going to have that convenience when it comes to your rifle, they are built around hikers, climbers, snow boarders, etc.
Another worth looking at, though again a bit pricey, is Mystery Ranch. They make a model built for heavy hauling called the Crew Cab that is something like the Transformer of packs. It's very popular with the back country hunting crowd because you can hack up what you shot and pretty much load up everything that will fit, and you can physically carry. Mystery Ranch uses an interesting base frame system that the bulk of their packs attach to.
Both brands can be found on ebay, though used ones are not plentiful. If you are patient though you can find them. I bought my Gunslinger there and got a great deal. It was in like new condition and if memory serves about $80-$90 cheaper than new.