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I imagine that if it goes "bang", it's considered a firearm, although I doubt that the CFC has it on their list. Given the short barrel, it would also likely be a 12(6).
As to ballistics, according to COTW, the 2.7 fired a 3 gr (!!) bullet at around 650-700 fps, for an estimated energy of around 3 ft./lbs.
Quote: " ... it might do to dispatch a trapped mouse or eliminate an overly aggressive cockroach. It is not practical to reload these small cartridges."
No ballistic info on the 3mm. Said to use a lead bullet rather than the 2.7's FMJ.