Can't use an ATV to transport a firearm anyway.
Rules of concealment as ruled by the State Supreme Court in Hamdan, Cole and Kieth.
1. The person knows the firearm is there.
2. The firearm is hidden from ordinary view.
3. The firearm is within reach.
The III Dstrict Court of Appeals in Alloy ruled that a person must comply with both statutes 941.23 and 167.31 and in order to do so must carry the firearm "out of reach" when in or on a vehicle.
It is impossible to carry a firearm out of reach on an ATV as commonly constructed. Therefore you can run afoul of 941.23 if you encase it so that it is hidden from ordinary view or of 167.31 if you carry it uncased.
Same situation applies to snowmobiles.
I implore you. Don't shoot the messenger. Complain to your legislative representatives.