I believe the relevant Iowa Code for you would be:
724.4 Carrying weapons.
1. Except as otherwise provided in this section, a person who goes armed with a dangerous weapon concealed on or about the person, or who, within the limits of any city, goes armed with a pistol or revolver, or any loaded firearm of any kind, whether concealed or not, or who knowingly carries or transports in a vehicle a pistol or revolver, commits an aggravated misdemeanor.
4. Subsections 1 through 3 do not apply to any of the following:
e. A person who for any lawful purpose carries an unloaded pistol, revolver, or other dangerous weapon inside a closed and fastened container or securely wrapped package which is too large to be concealed on the person.
f. A person who for any lawful purpose carries or transports an
unloaded pistol or revolver in a vehicle inside a
closed and fastened container or securely wrapped package which is too large to be concealed on the person or inside a cargo or luggage compartment where the pistol or revolver will not be readily accessible to any person riding in the vehicle or common carrier.
g. A person while the person is lawfully engaged in target practice on a range designed for that purpose or while actually engaged in lawful hunting.
all of it:
http://www.legis.state.ia.us/IACODE/2003SUPPLEMENT/724/