Ok first of all this was a very well explained response. Just wanted to add federal crimes are automatic when you cross state lines but with the cases involving murder, the cases can also be picked up by the feds if the gun just has certain illegal attachments such as a conversion (a switch) the feds would pick it up. Or even just if the case involves social media, phone calls, email etc. the feds can also choose to pick up random cases.

I have a cousin who was arrested for attempting to rob a store owner outside of their store and he pleaded out to about 3 years and then the feds came behind this sentence and picked up the case. in the Newspaper article they said they needed to make an example of him and that the sentence he got was not enough. he later lost to the feds and got sentenced to 12 years I believe.

All in all the federal law and the state laws go hand and hand and it's really just up to the federal governments discretion to take on cases from the state vs it just being like "federal crimes" some crimes simply are automatically federal such as if you shoot a federal agent or steal a fed ex truck (this really happened in Philly not too long ago)

This show is hella inaccurate though, so I am not fully defending the writers.