A very simple fix for cracks like that is Super Glue. I use the cheap stuff (2 or 3 bucks for three tubes) from Harbor Freight and have never had a re-crack.
The biggest point of ease is the way it weeps all the way in. You do need to remove the stock so it will not go all the way thru, so glue does not leak into the gun. Put masking or packing tape close to the edge of the crack so excess can be removed easily and refinishing kept to a minimum.
Put some in the crack, it will sink down, wait a few minutes put more in. Keep doing it till crack is full. Wait 5 minutes and raise level to higher than the wood . After you work it back down to flush the repair will look like a black line, can't be helped. As yuur crack seems to follow the grain that should not be a problem.
O Yeah, wear surgical gloves !! One little goof and your glued to the gun.