On examples of the real weapon I've seen, like the Flak38 the barrel was Parkerised, the compensator nut was dull steel, and the flash suppressor black painted (worn) over dull steel. So, for the barrel, semi matt black, with a hint of blue, a steel, or dark grey with a touch of silver for the compensator, and very dark grey, with grey steel 'wear', for the flash suppressor. When it's all dry, give the barrel and suppressor a coat of semi-matt clear, then, brush on some graphite dust from a pencil, or roughly sand the pencil and lightly rub it over the whole lot. On th inside of the suppressor, give it a run around with the pencil tip.