I used the Plumb Brown browning chemicals to do my rifles for years with good results. I did have issues with getting consistant heat and even color, but I worked it out by bead blasting the metal parts, degreasing them well, and getting consistant heat. If you look close under good light you can see the chemical work on the metal surface and be able to tell when and where you need to add heat. However, their new product doesn't work as well for me and I started using Davis cold brown or Homer Dangler's browning solution to do my rifles. They give a good dark even brown color after two or three days and is durable. Today, I use the Plum Brown solution to do screws and small parts.
Roger Sells