I think what you're effectively looking at is a rifled (Foster) slug. Turns out the rifling on the slug doesn't do much. The majority of the stabilizing effect

I think what you're effectively looking at is a rifled (Foster) slug. Turns out the rifling on the slug doesn't do much. The majority of the stabilizing effect on a Foster slug is from the fact that the center of gravity is well ahead of the center of pressure. Having an actual hollow all the way through the slug with rifling would probably have increased air resistance compared to having the rifling on the outside, while rifling on the outside would have better ability to apply torque to the projectile. So knowing that rifling on the outside doesn't work well, we can estimate rifling on the inside would do even worse.