Bbl Browning Soln,,Rust Bluing Soln,,,,They are just marketing names for the most part.
As long as what is in the bottle can cause the steel to rust,,it can be used to brown a bbl, or if you then boil it,,it can be used to blue the same bbl.
One of the most used cold rust blue solutions, Laurel Mountain Forge is actually marketed as 'LaurelMtn Barrel Brown & Degreaser'.
What ever, it's just a name.
L/M was originally targeted at the Muzzle Loader builders,, so the 'Bbl Brown' name.
Another,,a quick rust brown, Birchwood Casey Bbl Brown, was one of the best quick rust blue solutions around (untill they reformulated & elliminated the mercury bichloride from it)
As far as boiling/carding every coat,,or every other, or only after the last,,it's just different techniques.
As long as you get the desired results, that's what count.
There is really no 'one way' to do this.
