the color of guns

I simulate gunmetal by priming with a dark gray primer (very flat).  Then I dry brush some Testors steel  over it.  The more worn/used, the more steel I put on and in a random pattern.  Just a very light dry brushing- just a hint of metallic color.  Also, there is a very great difference in the appearance of "gunmetal" paints.  Try some different brands.  But I only use gunmetal colored paint for brand new or seldom used guns, otherwise I use the drybrushed steel trick.