a million rounds a minute

no shell. It's a development of the railgun. Only a metallic bullet moves. No powder, no container.

It could probably fire steel shot. If built properly, the barrel probably shoot a train of projectiles travelling in the barrel at the same time.

It's a version of the weapon in the Schwartzie's sci-fi movie Eraser.

It has to shoot 16666 projectile per second to make a million rounds per minute.

It's more akin an electromagnetic sandblasting unit...

Depending on the mass and end velocity of the projectile, it must develop a considerable thrust (just like a jet engine or a sandblast gun).

Physics rulez!