The first time I read rayiner's post, I took him to mean a firearm that is illegal for anyone to purchase or own, i.e. an "illegal firearm". Reading it again, I realized his phrasing was ambiguous and he could have meant "illegal possession of a legal firearm due to immigration status", hence my edit.

So basically, it could be a legal firearm possessed illegally, or a banned firearm, based on his wording.