I would probably take it apart and see how deep they go if I was worried about it. A lot of your older guns get these cracks over the years. Especially on a sidelock gun. But more than likely the wood is just drying out as your smith said. If you are really concerned about it, take it apart and see. And no offense, but I do find it hard to believe that a guy who builds rifles would say he doesn't know anything about SXSs. What is the difference between the wood on a SXS and on a rifle? He should know something. Ya think?
