The 76mm gun that replaced the initial 75mm on the Sherman tank was a modified 3 inch antiaircraft gun (possibly the one that was on US Navy ships) that was adapted for use as the towed 3 inch antitank gun and adapted to fit the Sherman’s turret. The 76 mm gun-armed Sherman, in the form of the M4A1(76)W was introduced during Operation Cobra. See these two Wikipedia articles:
https://en.wikipedia.org/wiki/M4_Sherman
https://en.wikipedia.org/wiki/76_mm_gun_M1
I assume this was on the E8 Shermans?
It was a much better gun, despite the mere mm increase in barrel diamter. I suspect the modest diameter increase simplified field upgrades for existing inventory?