Scorch,
I think the mismatch is a given with the receiver finished at 600 grit and no polishing or buffing being done. But the cost of the boiling tank is a good point. Since I have one, I've never tried boiling a part one-end-at-a-time. I'm not sure, with distilled water and stainless welding wire to suspend it, that it won't color pretty uniformly even if he resorts to that. But as you say, a dip at a hot bluing shop shouldn't be too expensive and will sure be faster and easier.
Jim,
Steam cabinets, I thought, were just the old way to induce the surface rust, not to do the conversion. You'd set up the cabinet and put a watchglass on the floor of it with a few drops of nitric acid in it to get fumes to mix with the steam, then pipe the steam in from a kind of kettle. The rusted part was still submerged in actively boiling water in a tank for the conversion, AFAIK, but I'm prepared to be wrong about that. I never built a steam cabinet. I don't see why a hot enough stream of steam might not do the job if the presence of air doesn't just make the red rust deeper. It would take it longer than submersion to heat the barrel, but it might work, especially if you supply steam fast enough to displace the air in the cabinet.
Bill,
I've never tried bluing without the water actively boiling around the surface rusted part. Seems to me most of the conversion is done in 5 minutes under boiling water, but I recall I last wound up settling on boiling for 15 minutes to be sure conversion was thorough on thicker steel parts that took longer to heat up. Figure that once the flashlight shows the red is gone, you want to double that time.
If Jim is right that a steam cabinet can achieve conversion, then that's what I would make, in your shoes. The water is distilled when you boil it, so that part's taken care of. Just suspend the barrel vertically in a tube and pipe the steam in from the bottom. Let me know what happens and how long you end up having to leave it in there if that works out for you. I think PVC would work in that instance, since there is no pressure with the steam continuously venting at the top. Just be sure you have enough flow to get to the barrel temperature to the boiling point. I don't know that it works well if it's cooler than that. It's not like Parkerizing.