When land artillery was pressed into naval service

Were land cannons ever mounted on ships in the same improvised way that naval guns were used ashore?

Yes; available land weapons could be adapted for fire from ships, although several of the individual examples recalled here remain uncertain.

Available land weapons have sometimes supplied improvised firepower afloat. Gustavian chambered demi-cannons on galleys and small warships are one example recalled here, with transported tanks on merchant decks offered as a later analogy. Several other anecdotes remain uncertain: the alleged original army role of a Soviet recoilless gun is explicitly questioned. The Ottoman cannon account also moves between a claimed shipboard mounting and firing on the British fleet at the Dardanelles, without establishing a consistent mounting history. These qualifications matter when using the examples, even though the general possibility of adapting land weapons to ships is clear.