The Royal Armament Research and Development Establishment at Fort Halstead designed a new 120 mm rifled tank gun in 1957.

The British Army argued the modifying the turret would be impractical and that a smoothbore gun could not fire HESH rounds.

[3] During Operation Granby an L11 on a British Army Challenger 1 scored the longest tank-to-tank "kill" in military history, when it destroyed an Iraqi T-55 at a range of 4.7 km (2.9 miles) with an L23 "Fin" round.

[6] Unlike most Western tank weapons which fire a single fixed round, the projectile and propellant are loaded separately.

This required the obturation to be provided by rings in the breech rather than the cartridge case, as in fixed rounds and 125 mm separate-loaded ones.

High explosive squash head (HESH), smoke and other rounds used a hemi-cylindrical (i.e. a cylinder sliced in two lengthways) charge (the L3).

The projectiles for this ballistically matched those for HESH rounds fired from the main armament out to 2,600 m (2,800 yd), at which point the tracer element burned out.