The Plum Brown finish is very durable but as was said, it is difficult to get an even coating on large objects like barrels.
Plan on using at least 3 and more likely 4 coats to get it evened out.
The biggest problem is getting uniform heating on the large pieces. If the heat isn't high enough a coating of copper will form which has to be sanded off. If it is too hot, the solution will instantly boil leaving crusty deposits. Even after washing these off, the area will not brown to the same color as the rest of the barrel.
If someone asked me for an opinion, I would say use the Plum Brown for the lockplate and other small items and use one of the slow rusting methods for the barrel.
Oh, did I mention the fumes given off during the application of Plum Brown contains some really bad smelling stuff in it? Do it outside or in a very well ventelated room.