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US 3in was an 'legacy' gun, later the US built 76mm gun (for AFVs and as AT towed gun), both capable using same ammo as the 3in AAA? Again, equivalents in power with German 75mm L43/46/48.
The US 76mm used the same projectiles as the 3in gun but used a shorter, smaller case operating at a higher pressure to get the same ballistics.
An expensive upgrade as it involved much cutting and welding of metal by hand. I wouldn't be surprised if Firefly Sherman total cost (original tank price plus modifications) was as much as a German Tiger tank.
To make 17 pounder cost effective the Chrysler tank plant would need to design turret and hull for that weapon and mass produce them that way. IMO that's a fine idea but Chrysler needs to begin design work for Sherman / 17 pounder variant during 1942 so they will be rolling off assembly line by end of 1943.
If that were true most Sherman tanks would have 105mm main gun. About 3 times as much HE as 75mm shell. Or perhaps 90mm/40 cannon which provides a nice increase in HE payload and adequate armor penetration.
I think it's more likely Gen. McNair and others like him were afraid to risk failure (and his General's pension). So they stuck with a tank/gun combination which was adequate and already in production.