Watch out for the militia boogeymen.
Those pesky militia extremists might even conspire to rig the NICS system to facilitate the purchase by felons of weapons to arm Mexican drug cartel gunmen. Or they could make evidence disappear at the murder scene of a Border Patrol agent. They might even deport murder suspects so that Senators can't interview them. They could even, Heaven forfend, pay one-armed cartel snitches to move the arms across international borders.
Why would the FBI be upset about that?
Because they don't want the competition.