Interesting question! Hmm...I have a CVA kentucky pistol the barrel of which was never "treated". I just left it the way I got it...in the white. Now, I don't know if this would work...but what I've done with knives to give them a patina is take a paper napkin, saturate it with vinegar, wrap the blade with the napkin and let it sit for a few hours. The longer it sits..the more "patinized" it gets. I would not recommend letting it sit for longer than a couple of hours without checking it though. Now, if this would work with a barrel in the white? I dunno. I will say this though...with the vinegar treatment the finish I have gotten is not an even one as with a normal bluing treatment. It is more of a spotty or uneven finish..some areas are darker than others. I kind of like it. I do not know if this it towards everyones liking, but I figure if one let the barrel or the knife just "age", I don't think it would have a very even finish either. Seems to me, with mustard, what does the patinizing is the vinegar in it.
just my two cents....