In most wargaming army lists I am familiar with, there's a ratio imposed on fielding war machines. In Hail Caesar it's 3:1 for units to machines. I always figures that a scorpio must come first and I have two of them so they got built first. Now that I have eight cohorts completed, I figures I'd stretch to my onager model ... so here it is.
This little ripper is a Warlord Games model and like so many war machines manufactures and supplied in our hobby, it doesn't come with instructions. Well I didn't get any. Thankfully I have plenty of reference resources and there are a lot of very detailed drawings available on the web ... so no problem.
The model comes with three crew (I suppose that's obvious) and a cast pile of stone shot. I tend to roll left over Green-stuff into little balls and my chance both to hand were the right size so I augmented them - dumb luck.
The model is all white-metal. A trick for next time is not to glue the throwing arm-axle before gluing the winch-axle ropes. Mine aren't quite a taut as I'd have liked them to appear.
Back to my original reference, if you haven't built enough cohorts to earn your onager, you don't have a real army - you have a mere detachment.