dispatch,
Gorilla glue will foam, but there's a solution. I use it in some places building model airplanes.
Put a dollop (~3/8" ball) in a plastic cup (the kind that comes with cough syrup, or a bottle cap. Let it set for about 10 minutes (it will foam because it sets up by drawing humidity from the air), stir it with a small stick until the foam goes down. Now let it set until it foams again and stir it down again. Now do it one more time.
While you're waiting for it to foam, smear a light coat of vaseline (or KY jelly if you're old enough to have some laying around) on both pieces, right up to the edge of the glue surface being VERY careful to not get any on the glue surface.
The glue should be like molasses in January - sticky and stringy. Now it's ready to use. Apply a LIGHT coat on one piece and lightly dampen the other piece. Clamp it lightly, just enough to close the crack. The foaming should be largely complete. If some oozes out of the crack, don't worry about it. Let it dry and it will peel right off with your fingernail. The stock may break somewhere else, but it will never break there again.
If you are doubtful about trying this on a gunstock, use two other pieces of wood of any kind the same size and go through the above procedure. Let it set for 24 hours and try to break the bond. Betcha . . .