Different makes of steel will leave more or less of the iron oxide on the dry patches.
When the bore is DRY, the patch is difficult to pull out. The bore grabs it. That is dry.
When I (tight) patch after the WD40 flush (not just a wet patch), blasting the excess out the vent or nipple seat,
that patch comes out perfectly clean - it is nothing but a WD40 wet patch. If you run 2 or 3 more dry patches down and
out, none of them will have any more oxide on them, until the bore is perfectly free of WD40, THEN by patch number 5,6,7
those patches will start to show more and more grey streaks, it is not fouling.
In the morning, if left outside, all of those drying patches with slightly black or grey streaks will be rusted, with no black or grey on them.