I try to copy the Y-movement of the recoil bone as new driver and paste it in the recoil driver of the turret barrel. That magical connection between these just doesn't happen. The movement o the bone in pose mode doesn't influence the barrel. The only thing I can see is that the driver box in unchecked and I can't check it again. What's (not) happening?
Check that you have the correct orientation of the bone in edit mode.
If its local Y is a different orientation, then you might think you are moving it on the Y but not.
Try moving your driver in all axes and see if this is actually the case.
You can correct the orientation of the bone in the edit mode, and it should work.
If that isn't what's happening can you post your file or a screen recording of you showing the process of what you did?
That wasn't the case unfortunately. I'm afraid I have now accidently completly deleted the driver and even saved it. I don't know anything about drivers so I don't know how to fix that. I tried appending in the barrel from the original start file so I would have the driver back, but now I'm missing all the parenting that was in place. Maybe I should just restart the whole exercise and see if the same problem occurs. Takes a bit more time but fixing mistake on mistake is just making the mess bigger.
Just out of curiousity, is there any course that explains what these driver things are?
Redid it and now it works! I went for a run, reset my mind and tried again. Maybe something was off with the parenting.