The deck gun was an important part of a U-Boat's armament.

This one fired 88mm shells and could withstand repeated immersion in corrosive salt water. The wheel on the gun's breech (right) allowed the gun barrel to be raised or lowered quickly. Deck guns would fire warning shots at unescorted merchant ships to get them to stop, and could also sink a ship instead of using the U-Boat's limited number of torpedoes. Although a U-Boat at sea was an unsteady gun platform, the deck gun was a formidable weapon, outclassing the armament of most Royal Canadian Navy vessels.

Breech Loading Naval Gun
CWM 19390002-558