yoop
To start with, shellac is not much good for waterproofing... Nor is any oil finish. But the real problem likely stems from a different treatment on the outside and inside of the stock. If the inside is glassbedded and it extends up the channel for a ways it is waterproof. The factory finish is a long way from waterproof.
The outside changes moisture content with the weather and the the inside stays put. That causes wood to move.
Two approaches that I have used individually and together are stiffening the wood by removing a lot of it from the inside and filling with stiffened epoxy, and really sealing the wood.
Hollowing out a fore end until there is little more than a shell left and filling the cavity with short sections of old fishing rods bedded in fluffed epoxy will make it stiff and completely stable.
The other option is to actually seal the wood completely. Epoxy and superglue are the only two options I have found that will do that... I do not consider a total refinish on a factory stock worthwhile, so I cannot speak to how easily or well it will seal the refinished wood.
As 1minute said, time is doing a lot to settle the wood down, but some just does not want to quit moving.
art
