I think I can picture what you're saying and think I may have had the same "issue". How vertical is the rifle between the belt and the shoulder strap? I found the more angled it was the more I had this issue, if I remember correctly. Also how much slack you have in the belt/rifle butt portion, the more it seems to roll. Which makes sense because it has that loose run of webbing down to the rifle butt. The closer to the belt the butt is, the less it can roll, right?
Short version, if you think about it, the rifle is essentially against your body. It can't roll in to your body so it rolls away from it. Depending on the factors above and the weight of your rifle, I can see where a compact/light rifle would carry the best in theory.
I bought a gun bearer for CO last year and honestly I just wish the Timberline (and all their packs) had water bottle pockets at the bottom of the bag that you could stick the rifle but in and I'd have carried it that way. I was neither overly disappointed or impressed with the gun bearer. YMMV.
