Several attempts had been made to further improve firepower by fitting a more powerful gun. In parallel with development of the Cromwell and QF 75 mm gun, a new Vickers High Velocity 75 mm tank gun had been designed but this proved too large for the Cromwell turret ring and left a shortage in offensive anti-tank capability. A prior requirement for a 17-pounder armed tank led to development of the A30 Mk VIII Challenger. Based on the Cromwell, the hull had to be lengthened and a much larger turret set on top to allow a second loader for the 17-pounder, a requirement of the older specification believed necessary for the larger ammunition. The very high turret of the Challenger was considered a liability and this led to experiments with the similar A30 Avenger version, an anti-tank version with an open-top turret.