Yeah, I don't believe it's the stock. Or at least I've never seen any evidence pointing in that direction in my rifles, whether the stocks are synthetic or wood.
Instead, I've noticed rifles that change 100-yard POI in cold weather are the same rifles that change POI with powder charges only a grain or two different when working up handloads. Usually they're rifles with relatively lightweight barrels.
However, have never seen 100-yard POI shift noticeably in any rifle when muzzle velocity changed less than 40 fps from 70 to zero. Of course, that doesn't mean it can't happen--but so far it hasn't happened in any of my tests. Which is one reason I'm pretty fond of cold-resistant powders for hunting here in Montana.
