I was just watching the video about this bug and I have an explanation for the jittering effect. If you take a look at the collision used for the turret and barrel, you’ll notice that the barrel’s collision encompasses the entirety of the barrel, including the portion of the barrel that is inside of the turret. The physics engine, during gameplay, is trying to push the turret and barrel apart such that one object will not be inside of another. Over time, the amount of force being built up results in the entire tank to fly off.
So, how do we fix this? The easiest, albeit ugly way, is to pull the barrel outside of the turret itself so that the collision meshes aren’t intersecting each other. It’s not really a good solution, but it is one. The better solution would be to remove the collision geometry from the barrel, then add a capsule collider that you can place and scale such that it only covers the barrel itself, leaving the portion that’s intersecting with the turret alone. Because the intersecting portion is so close to the turret itself, you would barely notice when the tank is hit in that section.