[P/O W. 'Moggy' Cattermole]

There's two "big guns" in the video, the large railroad cannon and a super mortar (can't remember the exact size, but I think the gun was 480mm and the mortar over 600mm). Both types were used in the destruction of Sevastopol , which had a very strong network of concrete bunkers, artillery pieces and even large calibre guns from the WWI Russian Dreadnaughts. The city was completely destroyed during the war...

Essentially those were the early-war German super weapons. While extremely devastating and outclassing a battleship in firepower, it turned out that both were too difficult to transport and they could only be used in prolonged sieges, and at sites that had railroad networks...

Two of the railroad cannons were used against the Allies in Anzio and hid in some mountainous tunels after each shot, to avoid being spotted by Allied airplanes (the reload time was huge).

I reckon the railway gun shown was Schwerer Gustav, all 800mm of its gun calibre.

The other thing about the super heavy weapons was that they could only fire a very limited number of projectiles before they had to be take back to the factory for their barrels to be overhauled.