I know the one with the oval bore fired a disc-like projectile. It was to be kept secret. It was called the Shuvalov System, and "Secret Howitzer Model 1753." There are extensive inscriptions on the barrel about all the officials who approved it, inspcted it, cast it, etc., more than I have time to write here. The barrel is bronze and weighs in at 491 kg., the bore is 95x207mm., barrel length 162cm.
The rectangular bore gun has cast iron barrel, is called an "experimental 3-pounder gun." It was cast in Olonets in 1722. From their catalog, pp. 89: "The cannon was intended for firing with three 3-pounder balls, all together, Because of this balls were wrapped all round with linen, placed in one row on a wooden tray (spiegel) and tied with a rope round."
Cal. 80x230mm. Weight of barrel 492 Kg., length 156cm,
Interesting.