To load or unload
In Wisconsin it is legal to transport a black powder firearm that is loaded with powder and projectile in a vehicle as long as the firearm is in a case manufactured to hold a firearm and there isn't a cap on the nipple or powder in the frizzen pan.
There are only a couple of ways to unload a black powder firearm and the easiest is discharging the firearm. You don't want to be shooting after dark during deer season in Wisconsin. The warden could magically pop out of nowhere and he will surely try to make a case against you for hunting after hours. Removing the cap or the priming powder at the end of the days hunt render the firearm safe. This is of the same theory as with modern firearms: "the safest firearm is that with the action open".
One word of caution, do not take a cold, charged, black powder firearm into a warm house. The barrel will likely condensate and your powder will get wet. After hunting I leave my 50 locked in my truck at a stable temperature.