How much jail time will you get for having an illegal firearm?

Depends on why the gun is illegal, If you are a person that has been convicted of a felony (a serious crime) then possession of a gun is punished by 5 years in a Federal prison. No probation, no parole, no concurrent sentence- and a fine of up to $10,000. Make that an unregistered sawed off shotgun, or an unregistered machine gun, 10 years in a Federal prison.