A gunsmith friend was helping me do something to a rifle I built and took too many file strokes on a sight and got into the barrel. His scrape looked about like yours. I put LMF on the bare metal scrape and let rust, it took a few days adding more LMF, rusting and carding but the nick rusted nicely and although not a perfect match to the already browned barrel you wouldn't notice it if I didn't point it out.
I get very dark rust with LMF without heat, like this. I am pretty sure these have been oiled with motor oil after neutralizing, the oil makes them darker.
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