looks awesome, though the chain links magically become more and less depending on pose and action

and chain behaves a bit like rubberband in action
Yeah. It's by no means realistic, lol.
I tried making it more like you'd expect it to work, with the chain links each being a physics joint and the ball hanging at the end, but it didn't work at all. Was either jittery or it fell in slow motion, depending on how I tweaked the settings.
So the ball is now directly attached to the end of the handle with a single joint. At first there was none of that rubber band effect, but it just wasn't very fun to swing... it was like the ball was swinging around on a solid bar of metal. I figured it was already unrealistic, so I might as well make it fun, so I added some springiness to the joint and tweaked things until it was easy to swing at different speeds and circumferences.
The magical chain lengthening is an intentional effect to keep it feeling solid. If I figure out how to do the original "realistic" design, I'll remove it, but I'm moving on to other things for now.