Gary, that's a really profound change and I guess I should feel lucky that our climate is as gentle as it is. I too brought a completely but just finished stock up from the shop, and hung it in the sun for many hours this afternoon, just to make sure the oil was set. I use Circa 1850 Tung Oil, and the finish feels dry, but I just wanted to kick it with sunshine. But our humidity here in the Great White North is much lower than in other places in North America, both inside and out. My stock did not change shape in the least.
But a few years ago, I brought a rifle to PA for Dixon's Fayre, and the stock took on moisture to the effect that all of the metal pieces were sunken in the swollen wood, making it appear like poor workmanship. I lost points because of it. Oh well! Once I had the rifle back home, the wood shrank back to it's ambient moisture content, and all was fine again. One wouldn't think it would make that drastic a difference, but there you go.