Well, maybe you've got something special. I have never heard of an Investarms rifle stock that wasn't made out of European Walnut.
You may be right and perhaps your stock is made out of something else but, just in case you haven't worked with walnut before, it is a very light colored wood with open grain pores when it is unsanded and unfinished.
Walnut also splinters quite easily and if it is sanded against the grain sometimes long splinters will break out.
Before throwing your stock away, use some 60 or 80 grit sandpaper and sand an area with the grain until it is pretty smooth. Then re-sand the same area with 120 grit paper. When it looks uniform take a wet rag and the stock out into the sunlight and wet the wood with a rag.
I'm betting it suddenly becomes the nice, dark Walnut you expected to see. It may even have multiple bands of dark grain running thru it.
Give it a try.
PS: If the stock was made from the 'heartwood' of the tree it may indeed stay light colored. In that case, several coats of an alcohol based stain thinned with denatured alcohol will darken it to a rich brown. If you really want to get carried away with stains you might also try a coat or two of Mahogany for a red/brown color.
I would only recommend staining walnut if it was indeed made from heartwood and had prooven itself to be too light after the "wet test" with water.
Any stain, if overused, will cover the woods natural 'grain pattern' and make the stock an uninteresting uniform color.