Over at the World of Tanks website, tank researcher Nicholas “The Chieftain” Moran has posted an article on Sherman tank turrets and guns. Specifically, he found some archival materials about an attempt to put a 75mm gun into the T23 turret (this is basically the same turret as the 76mm armed Sherman tanks). Oddly enough, they found that the 75mm gun did not fit. While this may seem counter-intuitive since the T23 turret was larger than the 75mm gun turret, the answer lies in the shape of the turret and the placement of the trunnions. The Chieftain uses this example to make the larger point that up gunning a tank turret is more than just a simple case of making sure the turret ring is big enough and jamming in a new gun.
One of the problems with Firefly was that the turret was a little cramped.
The 17pr is not a small gun to cram into a turret bustle measured in inches. US Army Ordnance had a crack at putting the somewhat smaller and lighter 76mm into the same turret, and Armored Force found it unsatisfactorily cramped and rejected it.
Another lesser-known issue for Firefly is that in order to be converted, the tank had to be the 75mm one with the smaller turret. This is partly why the 75mm Sherman remained on the production lines even after the US Army decided to switch to the 76mm. But it raises the question: why did they mandate use of the 75mm tank? Surely the 76mm turret, bigger as it was, would have been a better match, and reduce the limitations the Firefly was under. How could the British miss this?