If the crack closes up tight with the barrel out, I would use thin cyanoacrylate because it flows into the closed crack. However, you need to have enough wood around the cracked area that you can scrape off the excess glue. Another option would be to wait until you have stained the stock to fix the crack. Again, this assumes the crack will close up tight on its' own with the barrel out. Otherwise, I would use Titebond III and clamp the crack closed to dry. If you have a complete break, a stock broken through the wrist or lock mortice, that is the time to use epoxy.