Regarding pushing the rifling to the outside of the barrel. The bullet would have to create enough pressure to exceed the yield strength of the barrel steel. The non yielding bullet would be pushing on the lands, and stretching the metal in the grooves. Assuming a 6 land barrel, the outside would have a slight hexagontal shape. You would also have an oversized bore. If one were to shoot a bullet hard enough to deform a barrel in this manner, the pressure required, would probably damage the action of a double rifle.

Dave