The difference between the Danish piece shown and the "Shuvalov's Secret Howitzer" is that the latter was apparently designed to fire a disk-shaped projectile. The bore cross-section was constant along the length of the barrel. The Danish piece must have an ellipsoidal-conical bore, in other words, the bore shape you see at the muzzle is the same back to the breech, but the width of the bore narrows constantly from muzzle to breech.