Cerakoting done the right way!
I´m pretty sure you have seen a lot HK416s in the Dark Brown color and I´m also pretty sure you were not so happy with the finish at all compared to the real deal! CRW Airsoft may have you covered then.
In this video, you see their cerakoted version of an 416D GBBR (we don´t know which brand it is, probaly ViperTech) but this finish is nearly perfect. If you´re looking for perfection, this one is right on point.