The Ordnance Quick-Firing 6-pounder 7 cwt, or just 6 pounder, was a British

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 57 mm gun, their primary anti-tank gun during the middle of 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...

, as well as the main armament for a number of armoured fighting vehicle

Armoured fighting vehicle

An armoured fighting vehicle is a combat vehicle, protected by strong armour and armed with weapons. AFVs can be wheeled or tracked....

s. It was first used in North Africa

North African campaign

During the Second World War, the North African Campaign took place in North Africa from 10 June 1940 to 13 May 1943. It included campaigns fought in the Libyan and Egyptian deserts and in Morocco and Algeria and Tunisia .The campaign was fought between the Allies and Axis powers, many of whom had...

 in April 1942, and quickly replaced the 2 pounder

Ordnance QF 2 pounder

The Ordnance QF 2-pounder was a British anti-tank and vehicle-mounted gun, employed in the Second World War. It was actively used in the Battle of France, and during the North Africa campaign...

 in the anti-tank role, allowing the 25 pounder to revert to its intended artillery role. The United States Army

United States Army

The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...

 also adopted the 6 pdr as their primary anti-tank gun under the designation 57 mm Gun M1.
Limitations of the existing 2 pounders were apparent even as the gun was first entering service, and an effort was made to replace it with a much more capable weapon starting as early as 1938.