TOBY458,
That means your .458 CENSORED/2.8" Kimber chamber has a non-SAAMI chamber.
The longest load possible using 2.8" brass with that bullet, seated on the fourth/last cannelure is ~ 3.980. With 2.5" brass it is ~3.680" COL (Cartridge Overall Length = COL, not COAL),
as previously illustrated...