The shim is a metal plate, about 1mm thick. If the stock swells, the shims going to move with it ...won't it? It would be better to glassbed the action to minimize movement.
I have a beautifully figured stock on my Sako Classic 308. The grain ran at an angle trough the fore end. You just had to show it a damp cloth and the fore end would warp. One side of the barrel channel pressing on the barrel it's self. I cured the problem by inletting a small piece of aluminium I-beam and glass bedding it in place. Now the rifle's POI doesn't **** with any sort of weather change.
I also have a Ruger gunsite scout rifle in .308. Stainless with a laminated stock, similar in material to the FTP stock. I coated the inletting with marine spar varnish. Its been out in the rain, snow, hot weather and out in the cold to -5C with no sift in POI.
I'm going to use the spar varnish on the inletting of the FTP stock. I'll keep an eye on POI shift in different weather conditions and let you know how I make out.
If I'm missing something please let me know.
By the way, I didn't have to enlarge any of the holes for the mounting bolts in the stock.