There is a method of joining a stock extension that virtually eliminates the joint from opening caused by atmospheric changes. If you look at the stable stock extensions undertaken by some of the top line craftsmen on this side of the pond and we have an atmosphere that can change from dry to wet a couple of times a day, well it is a maritime climate after all. The methods only problem is that you do have to be able read the wood and orientate the extension correctly so its movement is in unison with the gunstock its self and be exceptionally good craftsman to get the joint correct.
