Possibly "bulleted blank", a wooden bullet to increase the gas pressure so something such as a Bren gun could operate, giving the right shape to reliably feed into the chamber, and depending on a muzzle attachment to shred the bullets.
One of the factors in all this is the cost of making the steel bullet. Casting lead is an old and fairly cheap technology. The steel bullets will need totally different machines. Even the gilding metal stage will need some changes.