Both Thompson Center and the Lyman Great Plains guns are stocked in walnut and both companies often use some very nice wood.
For instance, my GPR actually has some "curly" wood in the stock.
Unfortunately, both companies also have been known to use finishes that cover up the true beauty of the wood.
They do this to cover or mask areas that are too light or plain to be attractive but in the process, they cover up the natural figure the wood might have.
Tru oil by itself doesn't add any color to the wood but if the old finish is removed before it is applied, it will bring out whatever color is there in rich detail.
Of course the old adage "You can't make a silk purse out of a sows ear." does apply and if the stock does have some light colored areas, the Tru-Oil will let them show clearly.
The trick is, after the old finish has been removed and the wood is sanded, the whole thing will look very light in color.
To see what it really is going to look like after the final oil is applied, take the gun outside into the sunlight or hold it under a bright light.
Then, use a wash cloth and some water to apply a coating of water to the surface.
It will immediately darken to its true color.
If you see a light area still showing, consider using a water or alcohol based stain like Birchwood Casey Walnut, thinned with some additional water to lightly stain the light colored area.
Let it dry and then re-wet the surface to see if it needs more stain.
When it looks good, give it a light sanding with 220 grit sandpaper. (Do NOT use a grade finer than 220 grit). Then, apply the Tru-Oil or other oil gunstock finish.
While I'm wasting your time, walnut is an "open grain" wood. That is, it has millions of open areas making it appear rather rough.
If you like this look then just use the oil finish and apply several coats by rubbing it in until each coat is almost dry.
If you want a smooth finish with the grain filled in you can use a stock filler or better yet, use a "wet/dry", 220 grit sandpaper to apply the first couple of coats of oil. The sandpaper will sand off some of the wood, mixing it with the oil and this will end up being forced down into the open grain. After doing this a couple of times, when the grain is filled to your likes, you can use a small rag and the oil to rub in the final coats to make the final finish.