Rifle shells for a 12 ga smooth bore? Doesn't make sense. The bullet requires the lands and grooves to impart spin to keep the projectile from tumbling and for accuracy. There were brass 12ga shells, early black powder and even later smokeless shells were loaded in these before the advent of plastic hulls. Also, the weight of the bullet would be excessivly large needing a whopper powder charge to push, consider the .500 Nitro and the sheer bulk of just the chamber area I don't think that a standard 12ga shotgun could stand up to the pressure.As mentioned there are rifled slugs, which have fins built into the actual slug. There is a fine line between a hobby and insanity.