Well that killed the movie for me
; US only used the M4A2 at D-day, which also implies they weren't 76mm-armed.
The U.S. had 76mm armed M4s at the time of D-day, but none were the Duplex Drive (amphibious) versions. 1 in 4 Shermans in June of 1944 was armed with a 76mm cannon. Let me recheck my sources to make sure I don't have something confused here.
I know Wikipedia isn't the most trusted source, but it shows two variants of Shermans having 76mm at the time of D-Day:
M4A1(76)W Pressed Steel Car Company 3,246 January 1944 - July 1945
M4A2(76)W Fisher Tank Arsenal
Pressed Steel Car Company 2,915 April 1944 - May 1945
Now HVAP ammo, that's an entirely different matter. Rare stuff.