An even bigger problem for most cruiser tanks was the small calibre of their main gun. The first cruisers were armed with the 2-pounder (40 mm) gun. This gun had adequate armour penetration against early war tanks, but was never issued high explosive ammunition. This made the cruisers less able to deal with towed anti-tank guns, which became a major issue with the extended combat ranges of the Desert Campaign. the additional machine gun turret (as mounted on the Crusader) was no substitute for HE rounds. As the armour of German tanks increased British cruisers were up-gunned with the more powerful 57 mm QF 6-pounder gun, starting with the Crusader Mk. III (an interim move pending the introduction of the next cruiser tank). Early marques of what would become the Cromwell were also fitted with the 6-pdr but this gun still did not have a satisfactory HE round. The Cromwell as planned was to have a High Velocity 75 mm gun but the design was not sufficient to mount one and so decided that the new Cromwell tanks would be fitted with the QF 75mm (a bored out 6-pdr design that could take US 75 mm ammunition). The new 75 mm gun providing greater HE capability at the expense of some armour penetration, however it was still adequate to deal with the majority of German armoured vehicles. Part of the Cromwell's success was its high-power to weight ratio, provided by the adoption of the Rolls Royce Merlin engine as the Meteor, which delivered sufficient power for the Cromwell to have a maximum speed around on roads. The new powerplant enabled the tank to be far more heavily armoured and armed than previous cruiser designs.