The QF 4-inch Mk XIX gun^{[note 1]} was a British low-velocity 4-inch 40-calibre naval gun used to arm small warships such as Bathurst and Castle-class corvette and some River-class frigate in World War II, mainly against submarines.^[3]

It succeeded the higher-velocity World War I-era BL 4-inch Mk IX (typically deployed on Flower-class corvettes in the escort role). The Mk XIX fired fixed ammunition which was 38.5 inches (0.98 m) long and weighed 50 pounds (23 kg).^[3] The weight of the projectile was increased from 31 pounds (14 kg) for the Mk IX to 35 pounds (16 kg) for the Mk XIX. The high-angle mounting used for the XIX added some anti-aircraft capability and allowed it to fire starshells to illuminate the battle area at night.^[2]

• 
  Canadian gunners with fixed rounds
• 
  Starshell

• On HMAS Castlemaine at Williamstown, Victoria, Australia.
• On the parade ground at the Irish Naval Service Base, Haulbowline, Co. Cork, Ireland

• John Campbell, "Naval Weapons Of World War Two", Annapolis : Naval Institute Press, 1985, ISBN 0-87021-459-4

• Tony DiGiulian, Britain 4"/40 (10.2 cm) QF Mark XIX