It's already possible to do this, though with numerous, onerous restrictions (that I'd advocate against, obviously). For example, if you wanted to own a functional RPG launcher, that would be an NFA-registered DD, and each explosive projectile would be a separate additional NFA tax stamp. You'd also need an active FEL (Federal Explosives License). There's numerous other conditionals that apply, e.g., you wouldn't need the latter for inert projectiles for your RPG, or if the explosive charge was less that 0.25oz.