Federal prosecutors have no power or authority "to dictate" to state prosecutors as to the penalty
the latter may seek to have imposed on a defendant facing state court charges involving felonious posession of firearms. A federal felon in possession of a firearm charge will often come up in connection with other federal charges with which a defendanr has been charged, frequently involving illegal drugs such as possession of cocaine with intent to distribute.
A case like Felon in Possession could come in State court or Federal court. If it's only that single charge, and it's prosecuted in federal court, it's usually because the defendant has a terrible prior record or significant police contacts that are concerning. Also, under the Armed Career Offender provisions of federal law, a person would face a mandatory minimum 15 year prison sentence if they have three prior violent felonies or serious drug offenses at any point in their life. Often, the feds will take people who qualify under that provision.
The previous response is accurate that the feds cannot dictate to state prosecutors what to do. However, I have seen situations where the feds will dismiss a case in favor of state prosecution when the state prosecutor and the defense attorney jointly agree to make a certain prison recomendation to the court.