I tried something similar a few years back. I covered steel bolts with concrete. Sharpened the threaded section sticking out one end and the hex end was for small game. Unfortunately the shock wave running through the bolt on impact makes the concrete break up. You could use epoxy or epoxy covered wire but hunting sling ammo gets lost so easily it's almost not worth it.

A bullet or biconical shaped glande will fly point forward just like a bullet from a gun. If slung right that is. A glande with a sharpened point could be relatively small and still (hypothetically) kill medium sized game. That is my theory anyways. I believe lobohunter made broadhead glandes which proved how devastating a sharpened glande can be. The idea was basically to have small and larger game ammo in the same glande.

I don't sling spheres. They aren't as effective.