Just to elaborate on this theme a bit, as early as 23 March 1943, Gladeon Barnes "boldly claimed the T23 was designed so that “if desired, armor as thick as that used on the German Mark VI tank can be applied for a total weight of 37 tons” and that it could also mount the new 90mm gun. This memo appears to be the earliest reference to what eventually became the T25 and T26 programs." By that time, it was obvious the T20 and T23 programs with the 76mm gun were going nowhere, but for the next eight months or so Army Ground Forces and the Armored Force (which was downgraded to a "Command" on 2 July) fought a rear-guard action for the 76mm caliber. They saw the 90mm complete round as too large and heavy to manhandle in the confines of a turret, especially that of the Medium Tank M4, a view confirmed by the Ballistics Research Laboratory in tests the previous year. Worse, they were adamant that ammunition stowage not fall below 70 rounds, which was the minimum they believed acceptable based upon British and American combat experience. Instead, they advocated for increasing the propellant load in the existing 76mm cartridge as a means of gaining greater Mv. However, the gurus of the Ordnance Artillery Branch non-concurred strongly, claiming that in paring down the 3-inch Gun M7 tube and breech from 1,990 pounds to 1,204 pounds they had cut safety to a bare minimum. Never mind of course they had also reduced the propellant load from 4.62 pounds to 3.62 pounds, while reducing the chamber volume and ballistically matching the performance of the two guns. No, no actual testing to destruction was not necessary to discover if increasing the propellant load was possible.Richard Anderson wrote: ↑22 Sep 2020 01:46There does not appear to be any clear reason for that. My inference is they tested the German 7.5cm AP in April 1943...and by then Ordnance was already pushing for the 90mm, which Armor did not like, preferring a more powerful 75/76mm. So Ordnance did what they wanted to do, which is pursue the 90mm gun as the "simple" solution...and since 76mm would go away in 1945, there was no need for something similar to the 90mm AP T33 for the 76mm.Delwin wrote: ↑21 Sep 2020 21:59Which I find extremely odd. For sure production of ABCBC ammo would be much cheaper. OK - maybe not perfect option but penetration similar to German 75 mm KWK L48 should be achieved and even slightly surpassed -especially since both velocity and weight of the projectile of M1 gun was slightly higher. The other issue is that even HVAP ammo barely (400 yards ?) was able to went through front turret of Panther.
About the same time the design and production quality superiority of the German 7.5cm AP projectile over the American 75mm, 76mm, and 3-inch rounds was discovered, but again, Ordnance conveniently did not need to do anything to correct it, because the next generation of tanks would have a 90mm gun. Never mind again that shortly afterwards it became obvious that late 1944 would be the earliest such tanks could be in combat...obviously an inconvenient detail.
In any case, presto! At the end of 1943, all Ordnance's problems went away when first Devers and then Eisenhower at ETOUSA confirmed it accepted and required the 90mm gun as a future replacement for the 76mm, while at the same time the 105mm howitzer would replace the 75mm as the premier high explosive thrower. So obviously Barnes and the gurus of Ordnance were prescient compared to the stick-in-the-mud dinosaurs like McNair and Gillem.
Of course, this happy sense of complacency went out the window when Ike sent his July rocket to Marshal and the shit truly hit the fan. Instead of the 76mm being a perfectly good interim weapon until the 90mm was fielded, the issues with the APC projectile and fuse design and production meant it under-performed...and, worse, the same issues were found in the 90mm APC. So Ordnance's quick and dirty fix for the 76mm was HVAP, which was a good enough solution since by then it was confirmed the 76mm-armed Medium Tank M4 would be eventually replaced by the 90mm-armed Heavy Tank T26E3. Thus, they worked on both HVAP T30 as well as AP T33 for the 90mm.
Nevertheless, the dinosaurs tried again, in September 1944, by describing a requirement for a 45-ton medium tank armed with a 3"/76mm gun firing fixed ammunition weighing no more than 35-pounds (76mm APC was 24.55 pounds and 90mm APC was 42.75 pounds), but with a penetration of eight inches of armor at a 30°degree impact at 1,000 yards, using HVAP ammunition. The gun would be fully stabilized and would include a built-in turret rangefinder. Eventually they got it...in 1949 as the T91E3/M32. By that time improved production processes had also been finally introduced in the M339 projectile, which was AP rather than APC.