Marc,
Any radius is possible. Simply use a convex side milling cutter (horizontal mill cutter) and a horizontal mill or a stub arbor in a vertical mill and tip the barrels up. By using a cutter of just the width needed to cover the widest point of the rib, you can then grind any radius on the cutter desired and still not hit the barrels. I know not everyone has the equipment, but I'm just stating what is possible if you're willing to pursue it.