The Germans were working on a Leopard 2 - 140 quite a while ago.
Leopard 2-140
As the 1990s began, Rheinmetall began developing a 140 mm smoothbore cannon as a future tank cannon. This was intended to counter new developments in Soviet-bloc armoured fighting vehicles, most especially persistent rumours that the next-generation Soviet main battle tank would be armed with either a 135 mm or 152 mm cannon. This program was contemplated as the third stage in the KWS program of modernizing Leopard 2 tanks. KWS I was the replacement of the L44 120 mm cannon with the 55-calibre model, KWS II was a modernization program that became the Leopard 2A5, and KWS III was the development of a new turret including a 140 mm smoothbore weapon system and an automatic loader, which would have resulted in the reduction of the crew size to 3 soldiers. The final project design contained a lateral loading mechanism and had the main gun moved in the left turret side. Ammunition load for the main gun was 32 rounds, which were stored in a large ammunition bunker, covering the full turret rear. Moving the ammunition out of the crew's compartment would have resulted in a higher survivability in case of a penetration. The planned protection level was to be equal to the Leopard 2A5 or better. Command and control of the tank was supposed to be improved by the introduction of the ISIS system in its latest version. The KWS III was not adopted then, but development continued on the 140 mm weapon system, with Rheinmetall coordinating with Royal Ordnance from the UK and GIAT from France. To test out the weapon's capabilities, the 140 mm gun was mounted to a Leopard 2. The tank was not equipped with the new turret armour of the KWS III improvement program, nor with an automatic loader, and it also still had the electro-hydraulic turret drive. To cope with the extra weight of the main gun, counterweights were added to the turret rear. The tests were partially successful, with the gun showing superior penetration power, but also some difficulties with the handling.