That sounds like an automatic rifle. He definitely shouldn't have that.
if you have the money, which it sounds like this guy did (retired pilot, owned 2 planes), you can legally purchase fully automatic weapons.
even if you didn't want to pay the 15K for the documentation/licensing required to purchase an automatic weapon, you can spend a couple hundred and buy a bump fire stock. basically turns a semi auto rifle into fully automatic. all perfectly legal.
i bring this up because at the end of the day you will never be able to outrun the technology (even if you outlaw all guns today, are you also going to outlaw 3d printers?
you'll never be able to overturn centuries of legal precedent affirming right to gun ownership or the gun culture - it is ingrained too deeply within US society/history. we need to stop wasting time trying to take away guns and instead focus on:
1. Domestic terror intel - shame that trump cut the budget for this
2. More stringent background checks - I think the votes are there for this (you may get GOP votes in competitive districts/blue states)
Edit: computer cut out before I finished