superposed loads were experimented with by a few, the seal between loads sometimes failed and they all went off simultaneously, not a happy occurrence for the shooter.
modern attempts by 'metal storm' have proven more successful but not as far as i know, yet accepted by the military.
https://www.youtube.com/watch?v=wKlnMwuCZso
https://www.youtube.com/watch?v=NLNuBq0NQJE