20 Inch Naval Gun
The 20 Inch (508mm) Naval Gun was a concept never followed up on in the real world, but it has appeared frequently in NS. Originally it was designed by a US Civil War captain as a shore battery. There were a total of three of these 20" Rodman Guns constructed.
In the RL WWII, the concept of shipbourne 20" cannon appeared, and was in planning for the "Super Yamato" class Imperial Japanese Navy superbattleships and Hitler's unnamed Plan Z monster battleship.
Numerous NS navies deploy guns of this scale and larger on battleships or larger heavy gun-carrying classes such as the [in]famous Super-Dreadnoughts, and the technology is considered well established by many present and near-future nations.