ΔE 28.5, loose: different enough that you should pick on purpose. Heavy Armour is noticeably darker than Leadbelcher.
Can I use Heavy Armour instead of Leadbelcher?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Heavy Armour is noticeably darker than Leadbelcher.
Can I use Leadbelcher instead of Heavy Armour?
The same difference applies the other way round: Leadbelcher is noticeably lighter.
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.