Given the reluctance of anyone to pursue such cases the headline alone raises some flags. So it seems our miscreant had made threats to shoot a security guard and burn down a building at a local community college if they didn't re-enroll him. This got him reported to the local cops who picked him up for evaluation.
During the evaluation it transpired that he had an AR pattern rifle and a Glock pistol along with "multiple large-capacity magazines for both firearms".
So now all the DA has to do is prove that he acquired those mags after they became illegal back July of 2013. Possession of these mags is legal if you had them before the law was passed.
Unless the mags have a "made on" date on them it's going to be hard to make that particular charge stick. Meantime the whole thing follows the DA's standard practice of charging people with everything in the book, then offering to reduce the charges to what he thinks he can make stick on a good day.
Meantime I'm going to take this as an example of how big a moron you have to be to get that kind of attention from the authorities.