I do a variation on the techniques described.
First (after cleaning up the weapon and drilling our the end of the muzzle), I paint all metal parts Humbrol Satin Black (Humbrol number 85).
When the paint is dry, I rub a No. 1 Soft pencil lead on a small scrap of sandpaper, then apply lightly with my fingertip around the areas that wore the heaviest: around the muzzle, the trigger guard and trigger, the moving parts of the action and the sight (The parts soldiers touched most often.).
Then I carefully paint the stock with a light tan or gray acrylic paint. When this is completely dry, I work it with a dark brown or reddish brown oil paint. With a little practice, you can replicate woodgrain.
BTW, I paint the metal parts before the wood because I think it is easier to touch up if I goof and slop a little paint.