I’ve seen a few mentions of the mice of Sprucetuck using gunpowder (though I have seen any mention in the book myself), and I thought…why not cannon, mortar, and arquebus.
They would be rare, and very hard to make. The craft would (or should) be secret. Cannon and mortar could be used in military action, and the arquebus might be used in personal combat.
As for the stats of the weapons, cannon and mortar need not be statted, as I would suggest that they would add dice to a Militarist roll as supplies. The arquebus on the other hand could simply be Deadly and Missile, perhaps with Clumsy as well.