The math and simple logic tell you that no matter who did it, the easy availability of 'serious'guns like assault rifles make it more likely that they will fall into the wrong hands.
If there are 100 'legally' owned the odds are less of them getting stolen or ILLEGALLY sold, than if there are 50,000 legally owned. If there were detailed records of who purchased each one and who sold it to whom, and jail time for misuse and/or illegal sales, there might not be a story like this.