Help needed building an onager

[Robert]

It's not really off topic, because in one of my previous posts I postulated the Romans carried their ammo on the cart with the onager. As with any artillery, weighted and matched shot will very much improve the gunnery. Even heavier and lighter munitions could be used to get the rocks to fall on target, as that would be one of two variables, the other being the amount the spring was wound. Nice round stone ball!