ΔE 10.9, fair: the same colour family, but not a straight swap. Gunmetal is a little darker, more saturated than Rough Iron.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Gunmetal is a little darker, more saturated than Rough Iron.
The same difference applies the other way round: Rough Iron is a little lighter, more muted.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Rough Iron on the colour wheel See Gunmetal on the colour wheel
All matches: Rough Iron equivalents · Gunmetal equivalents