Looks like a scratch. As you move away from the butt plate, the little hook at the end of the scratch starts to cut across the grain. A crack typically follows the grain of the wood. Also a crack tends to widen as it moves toward the less constricted portion of the wood, as in around the action cutout, or the sawn end of the end of the stock. The mark on your rifle appears to be of uniform depth and width throughout its length. Its a scratch
