Onager - range/size/load

[Nathan Ross]

the ballista was a gun but the onager was a howitzer

I was thinking along those lines myself. However - if it's correct that the Romans abandoned the stone-throwing ballista at some point in (probably) the 3rd century, using the word instead to describe an arrow-shooter, and changed to the onager for their stone-throwing, then surely the onager must have been able to shoot on a rather flatter trajectory as well.

After all, 'guns' are useful in warfare...