How realistic would 20-inch guns be on a ship such as HMS Incomparable?
Short answerThey are near the practical upper limit: possible in theory, but the barrel, turret, ammunition, and hull become extraordinarily large and heavy.
NotesThe answer treats 20 inches as the upper edge of practical naval artillery rather than a useful normal calibre. A 45-calibre weapon would have a barrel about seventy-five feet long before accounting for the machinery that moves it. Multiple barrels, their turret, shell handling, and a viable ammunition supply compound the weight rapidly. The result demands a ship on Incomparable's scale, making 18 inches the more credible heavy-gun choice.