Subtle whiff: replace the 120 mm L/44 M256 with a L/55 version using something such as this:

Interestingly, this was kind of investigated in the real world with the M256E1: To improve the lethality of the M1A2 main battle tank, the US Army funded the Advanced Tank Armament System (ATAS) research project to integrate a longer gun tube. The long tube is based on the German Rh-120 L55 tube, made by Rheinmetall, and is 1.3 m longer than the conventional M256 120 mm gun tube.