Hi,
I don't know any specific ones, but if i find one i'll PM you, but i think i would try getting/making a "Metal Shader" of approximately the right colour and then play around with the specularity and other settings to try and make it look better.
I am not that sure what type of "gun metal" you are after, there are so many from dull black teflon type materials to silver and gold, but I suspect you will be after that "dull grey" colour?
If so, I will need a shader like that for a project soon, so will keep a look out for you/us.

Regards Scraggy