Casemates have a reduced accuracy for their first shot when they need to turn towards their target, because their hull still rotates while the gun is already pointed towards the target. This means they shoot with moving multipliers (reduced accuracy and higher scatter), giving them a miss more frequently.
This topic surfaced shortly again in the JP4 thread, and I had a bit of time to test it. Unfortunately, I tested it within a mod where I played around with target sizes (e.g. I gave the T34 a target size of 80, T70 around 60-ish or so). Interestingly, the unmodded JP4 still missed about half of its shots (I counted about 20), which is a hint that moving modifiers are applied after capping the accuracy at 1 and the fix as described below should still work as intended.
If the unit is still rotating when shooting, we must make sure it stops before the shot. The easiest way is probably to delay the shot enough until the unit is fully rotated. For example, the JP4 rotates at 20° per second, and the unit is allowed to shoot when the target is 5° off the center. This means we need to delay the shot by 5/20 = 0.25 seconds.
I then increased the ready aim time (this is only applied when a new squad/vehicle is targeted) from the standard 0.125 seconds to 0.26 seconds (slightly above 0.25 to make sure it works) and I did not have any accuracy issues anymore. I did not test for an awful lot of shots, but the first shot was suddenly reliable.
Since MMX brought up the issue of tracking I then took a T34 as normal and a vet3 T70 as a super fast target and circled around the JP4. I didn't see any tracking issues. As long as the JP4 kept up with its own rotation, it was able to shoot. Are there any other issues with this fix, or what was the reason it was not implemented?
Sorry for the tags @Sander and MMX. You guys seem to have either tested it yourselves or talked with people trying to fix it, so I'd be interested what exactly went wrong at the time so that the fix was not implemented.