BC is Birchwood Casey, the company that makes "Plum Brown" browning agent.
Plum Brown relies on the metal being heated to about 280 degrees F to make it work.
If the metal is much below 240 degrees F, the solution will apply a layer of copper that will prevent any browning to happen.
If the metal is much above 300* F the solution will boil and leave a mottled browning and crusts of the dried solution.
IMO, it isn't real easy to brown large things like barrels using PB but it can be done. It takes several coats. Wash the dried material off between each coat.
PB will look rather uneven after it is applied but letting it rest for at least 8 hours and then applying a coating of gun oil will even it out.
I use PB for small parts like lock plates, butt plates, trigger guards and nosecaps (if they are steel) and it works great.
Larger parts, sometimes not so great.