The M4 Sherman was originally equipped with a 75mm gun, that, while it performed reasonably well in North Africa, was found seriously wanting later in the war. A higher velocity 76mm weapon started to replace the older 75mm weapon, but this took far longer than expected, and Shermans in France and Italy found themselves coming up against Panthers and Tigers, against which the 75mm gun was simply inadequate. The article is about the end of 75mm gun and the switch to the 76mm.
Thanks but how does 1 mm make such a difference?