Some reflections on US WW2 tank gun development, might be relevant for AH purposes:
The way the Americans designed the more compact and lighter 76mm M1 from the 3" M7 gun was not purely through better technology, but also a reduction in certain requirements:
The 3" case was was designed with a huge safety factor, that is that they didn't fill it with the powder charge as much as they could. The 76 M1 vastly reduced the safety factor, so that it could carry a comparable charge within a smaller case and keep comparable ballistics. This also meant they could reduce the size of the chamber and breech, by 25% in the former's case. The barrel was also thinned out, but I don't know if it was compensated by better steels or if the barrel was just less overbuilt.
The 3" M7 actually had an impressive potential thanks to being so overbuilt. One could remove the distance wad and fill the case with more powder, and the recuperator and buffer were designed to handle 17pdr levels of pressure and recoil.
The Americans actually testing a slightly higher loading (108-115%). In the optimal configuration (where the case was still not filled to the maximum viable), it could increase the velocity of 15lb projectiles to 2800 fps instead of 2600 fps, and actually improve the reliability of case ejection. To the best of my knowledge, the Americans sadly kept the 2600 fps muzzle velocity. 2800 fps would have increased penetration by .5 inches for APC, and .7-.8 inches for HVAP, increasing reliability against a Tiger I from more angles and ranges, Tiger II sides, and a Panther's turret front and mantlet, which were not always easy targets for 2600 fps MV ammo.
This would also have put the 3" M7 just below the British 77mm in terms of penetration, as the latter shot a higher quality 17lb projectile at 2575 fps and penetrated .8 inches more than the normal M7 and 76mm.
However, as I pointed out, the 3" M7 was overbuilt to even handle the 17pdr's power. The Canadians concluded that the US could develop a supercharge to push the muzzle velocity to 3000fps. This could probably still be done with the existing barrel (while retaining good barrel life by British standards at least) and case, with some improvements to other parts here and there. The result would sit between the 77mm and the 17pdr.
The British regularly supercharged their rounds, generally increasing velocity by 200 fps or more for 2pdr, 6pdr, and 25pdr ammunition.
The American obsession with keeping multi-thousand round barrel life in WW2 (as opposed to their postwar guns) led to the M7 in particular being very underpowered for its size and weight.
However, even a supercharged M7 wouldn't be very efficient by late war, and this where some interesting facts come up.
The 3" was based on the T9 AA gun's barrel which nearly entered production in 1938, but was rejected in favor of the more progressive 90mm gun. The 90mm AA gun would eventually enter service in mid-late 1940, with some modifications in mid-1941. The 3" M7 started development at the same time the 90mm was entering service in AA form, in September 1940 or so.
This means that in theory, the US could simply have developped their new AT and tank gun around the 90mm from the start rather than the rejected 3" barrel from 1938. In practice, the 3" was so overbuilt/inefficient that the 90mm was very similar in bulk and weight, to the point where the US swapped the 3" for a 90mm in the M10 GMC and the M6 Heavy Tank with no issues other than those already present with the 3". Except that in exchange for somewhat heavier and wider ammo, the 90mm offered vastly greater AP and HE capability. The 3" barrel was not mass produced prior to the development of the AT gun, so there wasn't really the advantage of reserve parts being available.
To give some more perspective, the Americans did the exact opposite of what the Soviets did. When the latter introduced the 85mm 52-K AA gun to replace the 76mm 3-K, they cancelled all tank gun projects using the 3-K's ammo and ballistics and launched 85mm gun projects with 52-K ballistics, leading to the D-5T and Zis-S-53 of 1943-44. This was because they wouldn't produce any more parts to sustain a 76mm gun with 3-K ballistics in the future.
The Americans, meanwhile, chose to use a gun that didn't even enter service and had been already displaced, instead of the system that they were making the manufacturing capacity for. They couldn't even synergise with the 90mm industry until designing the 90mm M36 and the M26! All that for an inefficient gun design that was vastly less powerful and future-proof and only offered somewhat greater ammo capacity. At the same time the 3" was in development, the British were developping the 17pdr.
Thus, the choice of the M7 not only prevented an early service entry of a powerful 90mm gun in service (OTL introduction date of the M10 GMC since it could be designed with a 1940-41-designed 90mm from the start), but it led to the idea of developping a more efficient gun with the same ballistics (the 76mm), maintaining the wrong level of firepower in the future. The 90mm T2X could have been developped at the same time the OTL 76 T2X series were, since integrating the 90mm was a matter of upscaling the turret and strengthening the components to the appropriate level. Again, note that the British were developping 17pdr Cruiser tanks since 1941.
The US was a country with such a huge and safe industrial capacity that it could pretty much achieve what the British always wanted, except faster, but it was also the only country that actively denied itself such an increase in firepower.