What's your usual paint?
I undercoat brass parts ( and everything else, usually) with rattle can Tamiya Primer in whatever grey or white is to hand, then paint over the top of it.
Guns are tricky - there are various 'gunmetal' flavoured paints available, I've never been convinced by any of the ones that I've tried, but I haven't tried them all. My usual approach is a good coat of matt black, then rub the side of a 2B pencil over the raised parts, then buff carefully with a soft cloth. You could use a matt very dark blue instead, if you want something different.
There are also chemical blackening solutions that can be bought or made with relative ease; I've tried a couple of the commercial ones and haven't been all that impressed, but that might just be me.
No doubt others will have other ideas.