[concretemasonry]

The manufacture and quality of the molds will be important.

How you cn fill them is also very important.

As little water as possible, gradation control and variable vibration on a table and being able to change the orientation while compacting will be a big plus. You may have a "nub" left at the filling point that will have to be ground off.

If you do not account for the settlement and compaction if you want maximum strength. Under controlled conditions, you should be able to get 8,000 - 10,000 psi with the right aggregate gradation. If you just pour a typical wet mix, it wet, you will only get 3000 to 5000 psi, no matter what "goop" you dump in.

 

[concretemasonry]

I also like the idea of a steel core (in a joking way - ).

Can you imagine what happens when you drop a heavy ball with a high density core covered by a few inches of resilient rubber? If could be great entertainment like watching an uncontrolled huge golf ball bouncing and rolling around. - Much bigger and heavier than a bowling ball (3 to 5 times heavier) and more unpredictable.