I see there are more fans of ferric oxide than ferrous oxide here
As you know, smokeless powder contains its own oxidizer and will work on the moon if so called upon. I don't know if acid loses its magic in a vacuum. But on a custom made Douglas barrel I wouldn't want to risk it! Although if I could afford to take it to the moon...
Of course poorly washed nitrocellulose may have remnants of both nitric and sulfuric acid in the residue in any case. So a bit of Hoppes no.9 on a few patches is called for anyway.