Anybody wishing to build and test out an onager by firing it at their enemy (tax office, bosses house, school etc etc), can find all they need to know in this handy book.
The Art of the Catapult: Build Greek Ballistae, Roman Onagers, English Trebuchets, and More Ancient Artillery
http://www.campusi.com/isbn_1556525265.htm
It's a good read, I give it five FTFOTS (firey things flying out of the sky) out of ten on the AWOMDB (ancient weapons of mass destruction books) scale.