In America, the correct way, is to put the rifle in your rifle-rack in the back window, in front of God and everybody, loaded, but with an empty chamber, and if it's not actually deer season yet, the second-rifle position is required to have a fishing rod, with extra points if you have a chainsaw, or, optionally, a bale of hay in the bed of your truck. You should lock the truck if you leave while in the store or whatever, maybe stick the saw in the truck if you will be a while. And nobody will mess with it.