Maple (seasonally over a 12" span) moves about 1/4" radially, and 1/8" tangentially as the wood expands and contracts while it equalizes with atmospheric humidity. The reason for slotting the pins is to allow for seasonal. wood movement across the grain, both radially and tangentially. While wood doesn't get any longer or shorter longitudinally, there is always a little angle to the grain in a stock. Just the waves of curl create a little bit of an oblique angle that makes the stock longer and shorter with the seasons. If you want to measure it, put a micrometer on your muzzle end cap and the muzzle crown during the different seasons.
Case in point;
I have a 48" barreled gun that will be .005" in September and .06"in late February (muzzle cap to barrel crown setback). When I first built the gun (finished in late summer) 5-6 months later, I couldn't get the muzzle end wedge out (in the late winter), so, I had to wait until June to get the wedge out to slot it some more. It wasn't a problem for the wedges closer to the lock, but it was for the furthest one. If I didn't slot the holes I think eventually it might have caused the stock to crack. So, the further out you go from where the barrel is really locked in there (the lock and tang bolts) the greater the cumulative wood movement. (I suspect the butt got a little longer too, but that didn't cause any fitment issues.)
