I make a lot of period furniture with painted finishes. A particularly effective one is faux tiger maple. Honestly, it can produce a tiger maple you would swear is genuine, even from only inches away. First, there is a basic understanding that the dark stripes ate really the softer wood since they ansorb any stain more. The painting method I use begins with a base coat of mustard yellow paint. You want some yellowish color as your base, and the reddish or brownish overall color comes from. You can purchase glazes at paint stores, but I use a historically documented ( 19th century/ as when guns were produced )glaze made with dry pigment suspended in white vinegar with some corn syrup added to serve as a binder. The best color is burnt sienna. You brush the glaze on over the base. It goes on pretty thinly. Then, before it dries, you use a piece of rolled up newspaper, pencil size or less. Daub it in the glaze, making the tiger stripes. Make a bunch of roll of slightly different sizes because once they get really soaked they are no longer effective. After it all dries, coat it with 2 to 3 coats of varnish.
On a gun, I would use an oil-based glaze, or even thinned oil paint. It doesn't look quite as real, but is still as close as the Lemans. My guess is these stocks were done in some variation of this method. Painted stripes never look real because they interact with the paint very inconsistently. Either too much or too little paint on the bristles, These brushes ( check out Johnson Pain in Boston, MA ) are generally used to stimulate long grain, not stripes that run across the grain. Even if this doesn't work for you, try it anyhow. It's a blast to do.
For what it's worth,
Norm Vandal