Hi Sean,
As promised I'm posting some of my thoughts from my own experience with rust bluing.
I've reblued many, many modern double barreled guns with rust bluing and several other types of guns.
Last winter's project was a more or less modern .50 cal underhammer which I'm not allowed to post here. (if anyone would like to see an example of my rust bluing, PM me and I'll get the pics to you.) I did PM the pics to Sean.
The process is labor intensive but straight forward. It's imperative that you have a few things.
A large tank and heat system for boiling the parts.
An ultra fine .0025" wire wheel Degreased and / or 0000 steel wool also degreased.
Pure water. Some folks use distilled water. I've always gotten away with using tap water. An unseen oil slick on the surface of your boiling tank can be your worst nightmare.
A good quality rust blue chemical. These two have worked well for me in the past.
Good lighting.
A day to devote to the process.
Important, Once your piece is degreased and ready to apply the first application of chemical, you must never touch the piece with bare hands. Latex gloves are advisable.
Never go over the same area twice with the applicator when applying solution. Don't scrub or rub the solution onto the steel but rather use long, even strokes.
Detailed instructions come with the solutions if you order them from Brownells.
I don't know if it's the way the Hawkens did it but it works for me.