Soviet heavy SP guns

[PoorOldSpike]

... accuracy has nothing to do with muzzel velocity. At least in rifles, my short barreled 45-70 gov't is extremely accurate even though the bullet is only travelling about 1,300 feet/second. They used this cal. to hunt bison at 1,000 yards and more with iron sights, and at that range a bison is a rather small target. My .338 cal. in a long barrel is also accurate travelling at 2,300 feet per second, I cannot say which one is more accurate.

Yes a low vel is fine if you've got all the time in the world to estimate the range,calculate the wind speed and direction, adjust the sights then fire against a stationary target, (remember the assassin in Day of the Jackal carefully zeroing in his sights by popping at a pumpkin?)
But in tank battles there's no time for that.
For example suppose a Pz IV armed with a long 75mm L/43 (740 mps) spots a moving tank at 1000 metres, the gunner simply lays his crosshairs on it and fires, and the shot zips in a fast horizontal line with negligible gravity drop and probably hits.
But if the gun was a short L/24 (385 mps), the gunner could not simply lay his sights on the target and fire, he'd have to first estimate the range and elevate the gun to fire at a point just above and in front of the moving target, hoping that by the time the shell arrived in its slow arcing trajectory the target would still be there.

By contrast, what a luxury for tank gunners to be able to simply lay their crosshairs on the target and fire! Only high-vel guns allow them to do that.
CM models this pretty nicely, that's why short low-vel guns miss like crazy unless the targets at short range.