how do i achieve a finish like this on wood?

You can achieve that look by removing the stock down to the bare wood. Then using a small propane torch, burn the areas that you want to look blackened. Just scorch the wood not let it catch fire! Then lightly sand it with fine sand paper to blend the scorched areas to the clear areas. Rub in some Linseed Oil and let it penetrate the wood. After rubbing off the excess a couple of days later, then you can put a satin finish to seal the wood if you like. Or leave it and add another coat of linseed oil.

If this is an old gun and you do this it will destroy the value of the gun...Otherwise...

Sand the stock Etc..with 3000 grit paper then apply several coats of linseed oil...sanding between each coat till you get the finish you want...

It appears to have only a stain applied. That plus a couple of coats of wax for protection should give you the effect that you are looking for.