A Three-Barreled Medieval Bronze Cannon, ca. 1470, Excavated in Croatia
 

Unfortunately there are no measurements given but based on its relative size shown in the first picture, I think it will be of ca. 50 cm length overall, the caliber being ca. 20 mm. The breeches are blown away, obviously by an explosion. The most important thing about this gun is that it was cast in one piece.

I am afraid that the spectacular theory of relating it with sketches drawn by Leoardo da Vinci is way too euphoric. We should acknowledge the fact that similar devices existed in Late Gothic sketchbooks before da Vinci.

Enjoy, and best,
Michael

The rest.