The key, as with any metal finish, is a good polish job combined with de-greasing. Heat the object to be browned to the point where the application just “sizzles” when applied. Allow to cool, wipe with shop type paper towels, rub down with steel wool and repeat as often as needed for the depth of color you want.
https://www.amazon.com/Birchwood-Casey-Barrel-Finish-5-Ounce/dp/B0000C5153