I needed to add an extension of about 2" to a Brno 22H stock. Did it a little bit differently but learned a lot. I cut the stock on a diagonal from the left side of the butt plate to the pistol grip on the right, just missing the cheek rest on top.

I then glued a nice piece of walnut onto the stock and shaped it.
Scratched the cut line runing with the grain and filled the scratches with brown epoxy. Smoothed this out, made finer scratches and filled with black epoxy, smoothed everything out and the line has disappeared

I think I'm ready to d a shotgun stock only adding nice wood to both sides.

I have a few pics. I can Email to anyone interested in seeing / posting here.

FWIW --- John