The QF 4-inch Mk XIX gun was a British low-velocity 4-inch 40-calibre

Caliber (artillery)

In artillery, caliber or calibredifference in British English and American English spelling is the internal diameter of a gun barrel, or by extension a relative measure of the length....

 naval gun used to arm small warships such as Bathurst

Bathurst class corvette

The Bathurst class corvettes were a class of general purpose vessels produced in Australia during World War II. Originally classified as minesweepers, but widely referred to as corvettes, the Bathurst class vessels fulfilled a broad anti-submarine, anti-mine, and convoy escort role.Sixty Bathurst...

 and Castle

Castle class corvette

The Castle-class corvettes were an updated version of the much more numerous Flower-class corvettes of the Royal Navy, and started appearing during late 1943...

 class corvettes and some River-class frigates in World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, mainly against submarines. It succeeded the more powerful World War I-era BL 4-inch Mk IX

BL 4 inch Mk IX naval gun

The BL 4-inch gun Mk IX was a British medium-velocity naval gun introduced in 1916 as secondary armament on the Renown class battlecruisers and Glorious class "large light cruisers", but which served most notably as the main armament on Flower class corvettes throughout World War II.-History:The...

 in this role, and its high-angle mounting added some anti-aircraft capability and allowed it to fire starshells to illuminate the battle area at night.

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