What isn't answered in the article was how this idiot got his hands on 100 or more .223 rounds.

It is evident he didn't have a rifle to fire them from. It isn't cost effective to purchase them, do this stupid method to fire them, then sell the scrap.

Where did these rounds come from?

I'd be checking police reports for ammo stolen from local residents and seeing if this moron should do some time (after her recovers) for at least reciept of stolen property.