The Bofors 40mm Anti-Aircraft Gun

The Bofors anti-aircraft gun was developed by a Swedish company in the late twenties and in 1937 the War Office decided to buy 100 guns and 500,000 rounds of ammunition. This initial order was later followed by a license to build them in Britain. Production began before the outbreak of war but only 233 had been produced by September 1939, production was slow but rapidly increased to fill the increasing anti-aircraft requirements. The practical rate of fire is generally listed as 120 round per minute.

Unlike other nations such as Germany, the British never held smaller calibre anti-aircraft weapons such as the 20mm in much regard as while generally having a higher rate of fire which meant a greater change of achieving a hit they lacked sufficient power to guarantee severe damage to the aircraft. In 1938 the Royal Aircraft establishment calculated that it would require 4 ounces of cyclonite to destroy a modern aircraft wing. A 20mm calibre weapon carries around 0.425 ounces while a Bofors 40mm round about 2 ounces. As you can see the 20mm was quite underpowered, trails against a Blenheim Bomber showed that 20mm ammunition generally only caused severe damage if it hit a vital area such as the controls or fuel tanks. Hits from a Bofors gun were generally much more severe.

A semi armour piercing round was produced for the Bofors in 1941 followed by a full armour piercing round at the beginning of the next year. These rounds were to provide protection against enemy armoured vehicles such as tanks. Around a million of these rounds were produced by the United Kingdom during the war, I have not yet come across decent penetration figures yet, a May 1941 document gives around 25mm, 30 degrees, 1300 yards for what is probably the SAP round, the full AP round probably had performance similar to that of the 2 Pounder anti-tank gun.

Bofors Carriage Data

Cartridge Data

Projectile Data

Ballistic data

Production of Bofors Guns by year *Commonwealth data is from May 1942 onwards.

Sources - British & American Artillery of WWII, AVIA 22 456 - 514, Bofors Handbook