Russian 20 inch naval cannon

Not unique, the American Civil War Navy to fielded such weapons:
20"/10 (/10 probably) Dahlgren Smooth Bore of 1862
16"/15 (/15 probably) Dahlgren Smooth Bore of 1887

There was a similar smaller calibre too:
406mm/20 Pattern 1867
 
Not unique, the American Civil War Navy to fielded such weapons:
Yes, I knew about the U.S. weapons, but I had no idea that the Russian Empire was also doing research and prototyping in this regard. The biggest difference I can see is that the russian gun wasn't just for coastal defence, but to be installed in the Minin in a layout similar to the HMS Captain, before it was changed to 4 11 inch guns due to the obsolecent design of the 20 inch guns. When the Cap kissed the seafloor the plans were changed to the as-completed broadside armament. https://en.wikipedia.org/wiki/Russian_cruiser_Minin
 
It turns out that the Russians built not only a 20-inch, but also a 24-inch cannon! With 882 kg (1944 lbs) iron ball...
In general, the Russians had many interesting guns that were forgotten.
For example, the Indian "Jaivana" is considered the largest smooth-bore cannon on wheels in the world. The Indian "Zamzama" is considered the largest such cannon used in battle. Caliber of "Jaivana" - 11 inches, weight of ball ~50 kg, or 110-111 British lbs, caliber of "Zamzama" ~9.45 inches, weight of ball 80 British lbs, or 36.3 kg. But, Ivan the Terrible used in 1563 in Polotsk siege huge cannon "Oryol" (Rus. "Орёл", "eagle"), with weight of iron ball 2.5 pood, or 100 Russian merchant lbs, or, 90 British lbs, or 41 kg. By my calculations, caliber of "Oryol" was ~10 inches - is bigger than "Zamzama", but, smaller than "Jaivana". In 1757 Russian engineers Danilov and Martynov created "Unicorn" (Rus. "Edinorog", "Единорог") guns, hybrid between cannon and howitzer. Produced many variants, with 0.25, 0.5, 1 and 2-pood bombs. 0.5-pood unicorn was 6 inch gun in weight and range of 12-lbs cannons, and, can firing 24-lbs iron balls. Engineer Danilov writed about 5-pood unicorn. Standart Russian 5-pood mortar bomb was 13.5 inch in caliber. 5 pood = 81.9 kg, or 178.6 British lbs. And, 5-pood unicorn can firing with 118-kg iron ball - or, 260 British lbs. It was bigger than "Jaivana"
Book of Danilov:
In 1877 engineer Popov (creator of circular ships) projected huge armour vessels, with 16 and 20 inch guns. In 1883 Obukhov plant built 16 inch L/20 gun, weight of barrel 84 357 kg, 697 kg with 427 mps or 631.5 kg with 449 mps. I don't have data for a 20'' cannon, but if I use data for a 16'' cannon, I get the weight of barrel 164 760 kg, 1362 and 1233 kg shells.
 
Yes, I knew about the U.S. weapons, but I had no idea that the Russian Empire was also doing research and prototyping in this regard. The biggest difference I can see is that the russian gun wasn't just for coastal defence, but to be installed in the Minin in a layout similar to the HMS Captain, before it was changed to 4 11 inch guns due to the obsolecent design of the 20 inch guns. When the Cap kissed the seafloor the plans were changed to the as-completed broadside armament. https://en.wikipedia.org/wiki/Russian_cruiser_Minin

Well, many nations done this. You see, the heavy rifled guns were quite new - and not always reliable! - technology at this time, and nobody was sure, how good they would be. So, as a backup, the development of very large smoothbore gun was ordered: if heavy rifles would prove themselves to be more tricky than anticipated, Russian Navy would have something to use.

According to this, early designs for what would become the famous russian circular ships were planned to use 20 inch guns. Given that this was the 20 inch gun the russians had, I think it's safe to assume that this was the intended gun.

Well, it was mainly an option, too. The original project proposal mentioned both 11-inch rifles and 20-inch smoothbores.
 
t turns out that the Russians built not only a 20-inch, but also a 24-inch cannon! With 882 kg (1944 lbs) iron ball...
In general, the Russians had many interesting guns that were forgotten.

This is not a reliable source; author speculates very much without any actual proofs.
 
"The biggest difference I can see is that the russian gun wasn't just for coastal defence, but to be installed in the Minin"

Wasn't USS Puritan, launched 1864, never completed due to the end of the civil war, supposed to have 20' guns?
 
Yes, two Dahlgren Smooth Bore Cannons in a twin turret. To my knowledge these would be around /10 calibre ones
 
There was never a "calibre" listed for Dahlgren guns, just the bore diameter.
 
Apparently she was also designed to use the 20 inch guns

It was an option, yes. A fallback option, to be exact, in case heavy rifled guns would require too long to be perfected. Fortunately, our 12-inch breech-loading rifle worked perfectly, so smoothbore alternatives were abandoned.
 

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