What are the keys to getting browning to take well on a lock or barrel?
Prepare the surface first, but not too finely, since an overly fine surface can make it harder for the browning solution to bite in and start. After preparing the surface, follow the browning solution's instructions. If you are in a very dry environment, it can help to set up a damp box, an environment slightly more humid and a bit warmer than ambient conditions, to help the browning take.