I don't know if it's federal law, or only in some states, but there is something like an "exigent circumstances" clause to allow (legally) felon in possession.
I remember reading a court decision on an appeal of a FIP charge, where the court ruled that the man's brief possession of a firearm as unknown people (
who turned out to be police) were breaking into the house he was visiting was reasonable & he didn't break the law.
Once he saw who it was, he set the gun down & assumed the position.
But the court said it was legal & reasonable for him to respond to an apparent home invasion by taking up arms, as long as the possession was brief & for that purpose.
Notice also that he did not own or possess the gun other than those few seconds. It belonged to the relative he was visiting.
He was still arrested & had the expense & hassle of defending himself in court.