Rolls-Royce 40 mm cannon

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	Weapon: Rolls-Royce 40 mm cannon Type: Autocannon Country of Origin: United Kingdom Year Adopted: 1941 Caliber: 40x158mmR Action: Gas Operated Fully Automatic Overall Length (mm): 3350.0 Overall Length (in): 131.89 Weight (kg): 310.00 Weight (pounds): 683.43 Effective Range (m): 1500 m Effective Range (yards): 1640.42

The Rolls-Royce 40 mm cannon, sometimes referred to as the Rolls-Royce BH (Belt-fed, Hydraulic), was a British-developed aircraft autocannon designed during World War II for anti-bomber and ground-attack roles. It was a gas-operated, belt-fed weapon with a caliber of 40 mm, firing high-explosive or armor-piercing rounds. Drawing design influence from the successful 40 mm Bofors anti-aircraft gun, the Rolls-Royce cannon was compact enough to be mounted on aircraft, offering heavy firepower in a relatively small package. Its intended role was to destroy enemy bombers or tanks with a small number of high-impact hits.

Developed in the early 1940s, the Rolls-Royce 40 mm cannon was trialed on various aircraft platforms, including the Hawker Hurricane Mk IID and later marks of the Bristol Beaufighter. Although the cannon performed well in tests, issues such as recoil, rate of fire, and overall weight limited its adoption. Ultimately, the RAF preferred lighter and faster-firing alternatives like the 20 mm Hispano-Suiza or the American-designed 37 mm M4 cannon. As a result, the Rolls-Royce 40 mm saw very limited production and was never adopted as a standard weapon in British service. Only a small number of prototypes and test models were built.

Despite its limited deployment, the Rolls-Royce 40 mm cannon represents a noteworthy example of mid-war experimentation with large-caliber aircraft armament. The weapon's development illustrates the challenges faced in balancing firepower, weight, and aircraft performance in the rapidly evolving air combat environment of the 1940s. Today, surviving examples are rare, and the weapon remains a niche subject for aviation and military historians interested in experimental armament concepts of the Second World War.