Taofledermaus has done some experimentation with 3D printing of 12 gauge projectiles. They have had mixed results, but it is clear that there is promise in this approach. Combining the base wad with the projectile, while incorporating a hole in the nose for an embeded penetrator is one obvious possibility.
Making the projectile solid, with a hollowed base, could also strengthen the projectile and improve performance.
The slow motion video of the firing is fascinating to watch.