During the Second World War, merchant ships sailing on the sea were often attacked by bombers, and many merchant ships had installed anti-aircraft guns on board.  However, it is difficult to hit an aircraft with an anti-aircraft gun when a merchant ship is swaying on the sea.  After the war, researchers found that statistics from merchant ships that had deployed anti-aircraft guns throughout the war showed only a 4% success rate in shooting down enemy aircraft.  
Generally speaking, the cost of anti-aircraft guns for the above-mentioned merchant ships only accounts for a very small part of the total value of the entire merchant ship.
 the researchers believe that anti-aircraft guns on merchant ships are not worth the cost.