Hi,
A couple of ways of finishing the Spandaus:
Airbrush some Vallejo black surface primer 70.602- you can airbrush it straight from the bottle.
When properly cured, allow at least a day for this, airbrush Model Master Gunmetal again, straight from the jar.
The finish looks very like the real guns I have seen in museums.
Alternatively, immerse the brass parts in AK interactive Brass photoetch burnishing fluid.
You have to give it two seperate baths, and watch the components as they change from brass to blackened metal.
The top Spandaus ( modelmaster parts)are method 1, the lower jacket( WNW Hansa Brandenburg photoetch parts), is using the burnishing fluid method.
Hope this helps?