I would be interested in hearing what is happening to the previous rust blue jobs you've done. Are they rusting or turning a different color? Is the finish mottled or uneven? Or is the blue simply wearing thin prematurely?
Also, what process and rusting solution did you use? Was it a true slow rust process, or an express blue? Did you boil the tubes before carding, or did you use steam for the conversion step? What did you do to neutralize after bluing, and what did you use for a final curing oil. Knowing these details might shed light on how or why your previous attempts failed.
I agree with Buzz, it appears that all Model 21 barrels were rust blued originally. The barrels were joined with a tin/lead solder, so if all traces of the hot salts were not totally neutralized and flushed out, hot salts bluing could come back to haunt you. Your experience shows that sometimes it is necessary to vary the rust bluing process, because all barrels are not made from the same alloy of steel.
