The Firearm Blog (by way of GunPundit) has a collection of the tiniest blackpowder artillery ever made.
So if you don’t want to spend 9k at Dixie Gun Works to get a real cannon, you can always make a teeny-tiny cannon. Check it out for more tiny artie than you can shake a stick at.
Hmmm, I think it might be cheapest to make a volley gun out of shotguns…
So Lilliput has reached the Gunpowder Age?