Metal,
I only intend to use steam after the browning process is completed, then I will use steam to turn the brown to black. I have always boiled barrels in the past in order to turn them black, but I think the steam is maybe going to be easier and I don't need to worry about water quality. For one thing, I don't have to worry about the water getting into the barrels. With steam all I will have to go is wipe out the bores and clean them.
I have been boiling them after each browning application and then carding them on a carding wheel. I now intend to do several browning passes and then steam them and only do the carding last. This should speed up the process but conserve the blacking quality as it is. Bob