A Bushmaster is a semi-automatic rifle that looks sorta like an M-16. In military terms, an assault rifle is selective fire (full or semi-automatic). This wasn't an assault rifle in the technical sense but it matches the policial definition.

News reports say the rifle had a scope and a bipod, so it would be pretty accurate out to a few hundred meters. They say the car has a hole in the trunk and they believe he did many of his shots though the hole. That's pretty clever. Let's hope there's enough solid evidence to fry the bastard. According to this CNN article, the ballistics match between the rifle and the victims. There's a link to a picture of the Bushmaster in the article.