Challenger 2 tank is equipped with an L30, 120mm rifled tank gun from BAE Systems Land Systems (formerly RO Defence). In January 2004, Land Systems was awarded a contract to develop a new smoothbore 120mm gun for the British Army Challenger tanks. Under the contract, a Challenger 2 has been armed with the Rheinmetall L55 smoothbore gun, as fitted on the Leopard 2A6 tank, and began firing trials in January 2006.

https://www.army-technology.com/projects/challenger2/