This federal charge is interesting “possession of an unregistered firearm” because there is no federal registration except for firearms except short barreled rifles, short barreled shotguns and sawed shotguns.
A Molotov is a firearm under the National Firearms Act or NFA.
Therefore, the charge for having an unregistered Molotov, bomb, silencer, machine-gun, etc etc - is possessing an unregistered firearm.