I have used BC Plum Brown on a bunch of rifles. Results have been mixed. I used Laurel Mountain on my last rifle and the results were much better than previous rifles.
With PB the key is clean! Wearing ctoon canvas or equiv. gloves wash the entire barrel down with hot soapy water. When dry wash down with liberal quantities of lacquer thinner. Rewas with hot soapy water.
Do it outside in good ventilation. Heat the whole barrel. I used a propane torch. I played the torch back and forth along the barrel starting with a bottom flat. As the barrel heats up apply the agent. I have always used those foam "brushes" available at teh hardware stor for painting. The agent should sizzle as it is applied. As the first flat is getting done go to the next, overlapping the application of agent so you are essentially reapplying agent to what you just did as well as the new flat. You will have to keep playing the torch back and forth on the barrel while your are doing this. After about 1 hour you ought to be done with the whole barrel. Wash off the barrel with hot soapy water. Wash off again with either a hot solution of baking soda in water or ammonia. Wash again with hot soapy water. Reheat the barrel with your torch and rub down with 0000 steel wool soaked in WD-40. This should give you a smooth brown finish. After the barrel has taken as much WD-40 as it will take I usually heat the barrel and melt bees wax into the steel surface.
As I said, I've done a number of rifles like that and they turned out OK, if I had the barrel perfectly clean at the start. Nevertheless, I prefer the LMF agent, although I clean the barrel for that agent too, regardless of what LMF says about their agent being a combination agent and degreaser.