Yes, except for in a vehicle, motorboat or aircraft:
Chapter 167. Safeguards of Persons and
Property
167.31. Safe use and transportation of firearms
and bows.
(2) Prohibitions; motorboats and vehicles;
highways and roadways.
(a) Except as provided in sub. (4), no person
may place, possess or transport a firearm, bow
or crossbow in or on a motorboat with the motor
running, unless the firearm is unloaded or unless
the bow or crossbow is unstrung or is enclosed
in a carrying case.
(b) Except as provided in sub. (4), no person
may place, possess or transport a firearm, bow
or crossbow in or on a vehicle, unless the
firearm is unloaded and encased or unless the
bow or crossbow is unstrung or is enclosed in a
carrying case.
(c) Except as provided in sub. (4), no person
may load or discharge a firearm or shoot a bolt
or an arrow from a bow or crossbow in or from a
vehicle.
(3) Prohibitions; aircraft.
(a) Except as provided in sub. (4), no person
may place, possess or transport a firearm, bow
or crossbow in or on an aircraft, unless the
firearm is unloaded and encased or unless the
bow or crossbow is unstrung or is enclosed in a
carrying case.
http://www.atf.gov/publications/download/p/atf-p-5300-5/atf-p-5300-5-wisconsin.pdf