Throughout most of the 20th century, ships of war carried their guns in mounts or turrets, installations that enclosed the breech ends of the weapons and connected them firmly to the ship’s structure. In the earlier sailing navies, ships’ guns were mounted individually on wheeled carriages, each located behind a gun port.
The Constitution carries 30 24-pounder “great guns” on her main deck, 15 on each side. Each of these cast-iron cannon weighs about three long tons, is about nine feet long overall, and stands a little over three feet high on its carriage. When at sea, where a ship rolls from side to side, pitches up and down, and yaws laterally to port and starboard—often all at the same time—it was imperative that such weapons were moored securely.
Occasionally, the tackle restraining a great gun would break, or one of the bolts to which it was attached would pull out of the ship’s side. When that occurred, the iron behemoth would respond to the ship’s motion and become a battering ram, rolling back and forth, smashing anything in its path and crushing men like bugs. A gun on a rampage could, under the right circumstances, smash its way through the hull into the sea. Stopping it was an extremely hazardous operation and usually required more than one attempt, but so dangerous was the rampaging weapon to the ship’s safety that the injury or even death of a few men in getting it back under control was considered a regrettable but reasonable price to pay.
From this very perilous occurrence at sea comes that label for a person of erratic or unpredictable actions, the “loose cannon” or “loose gun.”