A slow rust blue solution I made up years ago used either a 50-50 or 60-40 mix of CONCENTRATED nitric and sulfuric acids. Degreased wire nails were dissolved in this in a crucible until the acid stopped consuming the nails. The reaction was violent and produced a lot of heat and splashing and was done outside. The spent acid was then cut with distilled H2O and used as the rusting solution.
Of course, this was left on the barrels and parts for hours to rust before boiling and carding, but this would not mean that conc. nitric or sulfuric was OK to use as an etchant.
You would be wise indeed to tread slowly and experiment on scrap barrels, or stick with solutions and concentrations and dip times that are proven to work. I would guess that the new Radio Shack stuff will work, but you may have to play with the dilution and/or dip time.
