In the form of an smg or pistol, having to change out your barrels to reload sounds really ineffective.
Picture a clip with the rounds lined up one after the other - end to end - instead of on top of one another. Thats the clip/barrel of this type weapon!
Suggest you got to post #23 and view the video at the Metal Storm site. Pretty damn impressive if you ask me! you may not want to throw too many rocks at the concept!