Practical Hints on Shooting, K. Paul Trench, 1887, Barrels
https://books.google.com/books?id=xrwUAAAAYAAJ&pg=PA20&lpg
Pour a drop of nitric acid of 1.2 specific gravity on the barrel, and allow it to remain there for a few seconds; then wipe it off with a piece of chamois-leather. On iron this leaves a light-grey spot, but on steel a much darker onealmost black.