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That was pretty much the argument of the time. The lower velocity 75mm shells could be made with thinner walls and carried a noticeable difference in HE content than the 76mm HE shells.
The 75mm was perfectly good against MK IV tanks and worked pretty well against Panthers from the side/rear. Turns out the 76mm didn't work so well against the front of a Panther anyway so maybe the difference wasn't that great. Perhaps the 76mm should have been adopted sooner but they probably would have wanted an even higher percentage of tanks armed with 105 howitzers for HE work.
Rather late even if it had entered mass production ASAP. The German 7.5cm PaK40 entered service during November 1941. German vehicles armed with this weapon entered service during the spring of 1942.Part of the 76mm problem was timing, it was ready for production, I believe, at th end of 1942
WWII tanks, especially American tanks, spent a lot more time firing at soft targets then at enemy armored vehicles. The German 7.5cm cannon was a superior all around weapon in addition to being available in large numbers relatively early.As far as being "late" goes the 76mm gun could go through the front of a MK IV at 2000yds. The German gu could penetrate the Sherman at 1500yds or less so at any practical battle range both were vulnerable to each others guns.
WWII tanks, especially American tanks, spent a lot more time firing at soft targets then at enemy armored vehicles. The German 7.5cm cannon was a superior all around weapon in addition to being available in large numbers relatively early.
I like the Valentine as a standard tank.