Each chunk of wood will be different I'm sure, especially those with a lot of figure, however wood just doesn't expand/contract over its length to any degree. Unlike metals which have well known thermal expansion numbers that are "written in stone" wood is a living thing even when it is dead. It is well known wood expands/contracts significantly across the grain, however my experience indicates it expands/shrinks very little in length. I have a 155 year old random length t&g maple floor in my kitchen, nailed not glued. Some boards are 16 feet long and some are only a foot long, the rest anywhere in between. The boards are all 2-1/2" wide. Some are straight plain grain, some are quite nicely figured, a few even have some burl in them. Winter brings us 30% humidity and floor temps around 60, summer brings us 90% humidity and floor temps some days around 90. The floor gets mopped from time to time and gets water spilled on it near the sink now and again. I never ever see the end joints open up at all anywhere, but I do see the width of the boards, as narrow as they are, widen in summer and become narrower in winter. So I doubt our stocks are changing length any appreciable degree under normal operating/storage conditions we ourselves could survive in. And unless they are soaked in water when the canoe tips over and the rifle is found on the bottom a couple days later I'd wager it isn't a problem. Wood properly finished even with that extreme water exposure likely wouldn't saturate the wood.
I do know the rifle barrel from zero degrees to 200 degrees changes length .050" or more (which could happen in the same day during a range event) and if you don't have room enough on either side of the pins the barrel will bow up and down with temperature change when the pins come in contact with the lugs.
