I've been lucky enough over the years to fire (and even own a few) military weapons and I've always felt that the use of metallic colors on the metal parts was wrong. Most military weapons are given a dull coating at the factory when produced. Very few are natural metal. The coating does a couple of things - first it aids in slowing down corrosion (rust that is) and secondly it cuts down glare and reflections (which can ruin your whole day if the wrong person sees it at the wrong time). It starts to look mettalic after you've lugged the darn thing around in the field for a while because the coating starts to wear off and you start haveing the metal underneath showing thru.
On US weapons from the early part of the 20th century to the mid sixties, the coating was a dark grey (most other militarys were aproximatly the same). When the M-16/AR-15 and M-60 were introduced, the coating changed to black w/black synthetic stocks. The Ma Deuce stayed with the grey. (Just as a side note, unit armorer's are authorized to use flat black enamel to touch up warn spots on the finish of the M-16, M-4, M-60, M-203, M-92, and the MK-19. They get re-Parkerized if they go to depot for maintenance)
I usually paint pre-Vietnam weapons with the flattest grey I can find and then lightly dry brush silver or white on those areas that would receive the most wear. For Vietnam and later, I paint the barrels and recievers flat black, the front hand guards semi gloss black (Model Master Chrome Black is great) and the rear stock with MM instrument black (its slightly grayer than regular MM flat black). Then go easy on the weathering.
For a civilian weapon, I would use the current MM gun metal and when I had finished the wood parts I would dip the whole thing in Future. You rarely see a non-shiny civilian long gun.