In the vehicle, cold, lock and protected. Good wipe down on the outside and a dry patch part way down the bore, in and out once. Never had a problem.
When in drop camp, we keep a small shelter for tack, packs, and misc out of the elements. Wiped down as if going in the truck and then stuff in a gun sock and left in that tent.
If it got wet in bad conditions, blow out the load, swab the bore, then dry patch it well, break down the gun and give it a good wipe down. Load it up and into the truck or tack tent.