How did the cops know he bought a banned weapon? That he bought anything at all?
Here's something I find easier to believe - NV gun dealer performs a background check, It pings local law enforcement in CA. All they have to do is wait for him to return, then raid his house.
This suggests that the BG check system is less than secure...
Maybe private sales and seller did not do due diligence else entrapped him.
That would require collusion on the part of the FBI. I thought they only had authority based on federal law. Have you ever heard of the FBI arresting people for speeding?