Best ask Alexandre. TMK, he is the first person to make such a weapon, but IMO you can't call a cannon, no matter how small it might be, a firearm. The dictionary definition says that a firearm is a portable weapon such as a pistol or a rifle that fires ammunition. Even a miniature cannon is not exactly portable, representing, as it does, a weapon that usually moves on wheels or skids.
I'd really like to see the real thing, rather that what my eyes tell me is a CG rendering.
BTW, Alexandre has also appeared on another forum on which I write. We want to see more of this work, right?