If everyone who wants to own (to keep using this one example) an AR-15 had to buy a CNC, that would be quite the commitment. If the current model of people buying many milling machines and selling their services is the only viable way to avoid that, then you've already seen the feds start to take notice. After all, if you can get word of mouth around enough to make money, you're making enough noise to be heard.
3D printing is presumably going to become ubiquitous, and require no skill or experience to operate and maintain.
