Finally got around to re-visiting my M-14 to try and improve the work on the metal parts. I eventually got a finish I'm happy with by drybrushing with silver, black gun metal.
I only wish I could replicate a wood look for the stock...
Don't know of one - it's something I use now and then, but there's no 'set rule' for doing it. It needs a bit of experimentation, using washes to alter tones in various areas, and sometimes the addition of dry-brushed streaks etc.