ΔE 26.3, loose: different enough that you should pick on purpose. Rust (Metallic) is a little lighter, much more muted than Steampunk Copper.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Rust (Metallic) is a little lighter, much more muted than Steampunk Copper.
The same difference applies the other way round: Steampunk Copper is a little darker, much more saturated.
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 Steampunk Copper on the colour wheel See Rust (Metallic) on the colour wheel
All matches: Steampunk Copper equivalents · Rust (Metallic) equivalents