I find it odd that no one has mentioned the possibility that he could have been a black market dealer. Suppressors, short barreled rifles, and fully automatic machine guns, are all purchasable in the US if you can file the right paperwork and be approved for those purchases by the BATF, under the National Firearms Act (NFA). This approval process includes the ATF having you on speed dial to show up and make sure you still safely possess said items, and aren’t flipping them on the street for a massive mark up.

While an NFA regulated suppressor might run you $1200 after taxes and fees, a suppressor on the streets without the paper trail might go for closer to 3-5k. Actual transferrable machine guns, due to their extreme rarity in the US, command prices from $10,000 to $60,000 dollars through existing legal channels, and again, could be sold at a massive markup without the baggage of a paper trail. Even firearms legal to own without NFA restrictions would command a sizable markup when sold off the books. And this is how gangs have armed themselves for decades, through dealers just like this.