I can tell you that I have had good luck doing something pretty simple.
If there is a gouge or tear in the wood, then I take a dremel tool and gently round the side edges of the gouge, so it looks like a long, irregular oval. If the long edges are ripped or torn, then I gently dremel those so the edges are smooth. If the gauge is real shallow, I actually deepen it a bit.
I then mix the walnut stain color with accra-glass, and fill the area, letting it be somewhat proud.
I let the accra-glass dry for a full week. Then I sand the area (and the entire stock) really smooth, down to 400 grit paper.
It doesn't matter that the walnut colored accra-glass isn't the exact color of the rest of the walnut stock. Since the filled area is rounded and smooth, or oblong, the end result is that the filled gouge looks just like a natural flowing grain in the wood. Wood stocks with lots of grain always have areas that look like this.
If the surrounding stock (after staining) is a little lighter than the repair area, then just put another coat of stain on it, letting it get darker until you get a closer match.
Make sure that you put a very smooth clear finish on the stock, so that the whole thing flows.
I have shown my work to other folks, and asked them to find the repair. Very often, they can't!
By the way, this is a great way to repair a stock which has been cut with a slot for a receiver sight, especially if the slot is does not protrude too far down along the side of the stock. Round out the edges, and make the straight lines slightly waivy with your dremel tool. The key is to make sure the area does not look like a rectangle or a square. The end result, after repair, looks great.
Hope this helps.
Mannyrock