If you do use Plum Brown, be sure the metal is absolutely clean with no oil, grease or wax on it. This includes fingerprints.
The metal must be heated to 285°F for it to work right. If it is much cooler than that, it will leave copper plating on the surface. Copper plating will keep the steel from browning so if it happens it must be removed by sanding it all off.
If the temperature is much hotter than 285°F, the solution will boil off leaving spots and streaks. In any case, even when it is working right, it will leave a crusty formation that must be washed off with water before you can reheat the part and apply more coats. It usually takes at least 2 coats to get an even looking browning.
I do use it for small parts like a lock plate or trigger guard. I quit using it on barrels years ago because it is difficult to get an even looking brown on large parts like that.
When you've got an even coat of brown on your part it will look much lighter than when the process is finished so this last step is important:
After the final washing and drying of the part, apply a heavy coat of oil to the surfaces. Motor oil, 3 in 1 oil, or any other kind of petroleum based oil will work fine. Let it set overnight to soak in. The next day, wipe off the excess oil and your finished.