Q-ships, also known as Q-boats, decoy vessels, special service ships, or mystery ships, were heavily armed merchant ships with concealed weaponry, designed to lure submarines into making surface attacks. This gave Q-ships the chance to open fire and sink them.
British First World War Q-ship HMS Tamarisk
Q-ships hid naval guns behind moveable or pivoting panels
USS Atik (AK-101)
USS Big Horn (AO-45)
An armed merchantman is a merchant ship equipped with guns, usually for defensive purposes, either by design or after the fact. In the days of sail, piracy and privateers, many merchantmen would be routinely armed, especially those engaging in long distance and high value trade.
HMS Alcantara and SMS Greif dueling at close range during the action of 29 February 1916
RMS Carmania sinking SMS Cap Trafalgar near the Brazilian islands of Trindade, 14 September 1914
Kormoran in 1940. During a single ship action on 19 November 1941, off the coast of Western Australia the German auxiliary cruiser Kormoran sank HMAS Sydney before being scuttled.