724.28  Prohibition of regulation by political subdivisions.
  A political subdivision of the state shall not enact an ordinance regulating the ownership, possession, legal transfer, lawful transportation, registration, or licensing of firearms when the ownership, possession, transfer, or transportation is otherwise lawful under the laws of this state. An ordinance regulating firearms in violation of this section existing on or after April 5, 1990, is void.
90 Acts, ch 1147, §9