Walnut is the preferred wood for gunstocks for centuries because of it's stability as well as it's beauty. Most, if not all, factory stocks are sealed inside as well as outside to insure there are no significant changes due to moisture. Unless you leave your rifle out in a ranging downpour, you probably aren't going to see a change in POI due to swelling or shrinking. And certainly not enough to "damage your barrel". My BS meter got pegged on that one. All the areas you mentioned are generally dry. The concern is overstated.The flip side of the issue is that some synthetic stocks get "floppy" when they get warm, causing accuracy to suffer.
By the way, welcome to the forum.