Really, the issue was that the weapon he used was an UNREGISTERED short-barreled shotgun. SBSs are legal, as long as one registers it with the ATF and gets a $200 tax stamp for it. (This is assuming that one's state allows such weapons. They're legal nationally, just heavily controlled by the 1934 National Firearms Act.)

The article says nothing about a prior record, but considering he wasn't charged with being a felon in possession of a firearm, odds are that if he'd filed the paperwork and got the tax stamp, he'd have saved himself the felony charge.