Yeah, the further away from your body center of gravity the weight is placed, the more you are going to have to lean forward to balance it, thus causing undue comfort. Hence the reason many packs have tensioning straps, to tighten things down and keep it closer to your back. By placing the rifle on the outside of your new pack, you'll be placing a good portion of the load further away than you probably should. This is why the Eberlestock packs have the rifle scabbard right next to your back and the rest of the gear is on the outside of that.
Geb