1722 Experimental 3-pounder "secret" cannon of the Tula plant.
Experimental 3-pounder cannon of the Tula plant, 1722, Military-Historical Museum of Artillery, Engineers and Signal Corps, St. Petersburg.
The page I pulled this from does not have a ton of detailed technical information, and information is, at a search glance, not coming up with much more, but here are some selections from the page:
The baptism of fire "secret" howitzers (as they began to be called) received in the battles of the Seven Years War, in battles with the army of Frederick II. In the battle of Gross-Jägersdorf, it is the secret howitzers that play the main role in the victory.
However, the "secret" howitzers did not last long in service with the field artillery of the Russian army. Still, the survivability of the barrel was lower than that of conventional guns, and firing cannonballs from them was almost impossible.