To avoid "double jeapordy" Feds will usually defer prosecution to a local jurisdiction in order to have a fall back case if things go poorly at a State trial. The other way is that they'll let the jusidiction with "the most teeth" take the first bite.
That's how the Army convicted a retiree a few years back of murder after North Carolina failed to convict. No double jeapordy because of two different jurisdictions.
Machine guns are awesome until you have to carry one.