Frederick County Sheriff and Gun Dealer Indicted for Scheme To Illegally Acquire Machineguns |...

Those might be tough cases for the federal prosecutor to win in court. The charges against both people rely entirely on whether or not the Sheriff was considering purchasing a machine gun for his department.
To convict the sheriff, the prosecution has to prove beyond any reasonable doubt that the sheriff had no interest whatsoever in looking at a machine gun as a possible weapon for his department.
To convict the dealer, the prosecution has to prove beyond any reasonable doubt that the dealer knew that the sheriff had no interest in looking at a machine gun.
The fact that the sheriff's department didn't actually buy any machine guns is irrelevant -- all the sheriff had to do was want to see one to decide if he should buy one.
The fact that the dealer rented the machine guns to shooters at his premises is irrelevant -- once he bought a machine gun to show the sheriff he can do anything else legal (and yes letting someone else shoot the machine guns is legal so long as the dealer is with them).
These cases rest on the intent of the sheriff and whether the dealer knew that intent -- if the ATF press release is correct, they are both guilty, but when the case rests entirely on one man's intent and another man knowing that intent, it should be hard to prove unless they were both stupid enough to put the whole scheme in writing or explain it to a reliable witness.

I doubt very much that the federal prosecutor thinks he can win either case unless he gets one of them to testify against the other for a deal - which is almost certainly what the prosecutor plans to do.

Wish more of this would happen.