muskyman is right about the gun once you load it. I have used a muzzleloader for 18 years and have never had a problem with the gun going off. 3 different guns. Guns loaded with blackpowder with #11 caps; pyrodex powder and #11 caps and pyrodex pellets and 209 primers. The gun NEVER goes inside of the house, heated part of a vehicle, heated garage or ANYTHING heated once it is loaded. Then the condensation builds up and you are in trouble. A few guys we used to hunt with put their guns in the back of a bronco during a hunting season. Well, both had problems with their guns going off that year. We told them why, but they didn’t believe us. So when it happened again later in the season, they had to believe us, because they coukld think of no other reason.
Keep them in the back of the pickup and in an unheated garage and you won’t have to worry when your deer walks by.
Another suggestion I would make, just in case, is if you are having any accuracy problems change your bullet and powder load. 5 grains of powder can make all the difference in the accuracy of a muzzleloader. On the range with a rest, 3″ groups or better should be expected out to 100 yards, with an inline.
100 grains is ENOUGH! go less, not more when experimenting. An example: With the sidehamers they say if you hammer is blown back up you have excess powder. Well, I was shooting a 295gr. bullet and if I put 100 grains in there it would blow the hammer back. So, what sense would 150gr. make?
Good luck, they are a blast!!