Sarge
I am pretty sure the 76MM gun on the m18 is the same as the 76MM they put in later shermans.
The Sherman started the war with a crappy low velocity 75MM gun that could not penatrate the armor of a Tiger or Panther even at point blank from the front.
The M10 Wolverine was the first tank destroyer to use the 76, and it was an adapted 3 inch naval canon.
They later, like in 1944 started to retrofit this into older shermans and then making some with that canon at the factory.
The difference in penatration power of the sherman 76, and the M18, is the ammo. They tank destroyer units got specail ammo, like APCR or something while the normal tank units had to use standard solid AP.