Onager - range/size/load

[Renatus]

Would a single-armed onager allow a higher trajectory shot from its sling than a stone-throwing ballista, perhaps?

A good many years ago, Eric Marsden came to lecture the Roman Army course organised by Durham University on Roman artillery. An old artilleryman, B. G. Harris, put it to him that the ballista was a gun but the onager was a howitzer and Marsden agreed.