A disc tumbler lock having a code range angle, the disc tumbler lock including at least: at least one tumbler and a guard tumbler, the at least one tumbler and the guard tumbler defining a keyway aperture there through; wherein the at least one tumbler includes at least: a key engaging surface defining a key engaging zone, the key engaging zone having a first radius defining the major extent of the key engaging zone, and the key engaging zone having a second radius defining the minor extent of the key engaging zone; wherein the guard tumbler defines: a restrictive aperture comprising a restrictive width having a minimum dimension less than or equal to the sum of the first radius defining the major extent of the key engaging zone and the second radius of the minor extent defining the key engaging zone multiplied by the sine of one half of the following: the code range angle of the lock, minus the angular measure between a last code angle for the lock and a second to last code angle for the lock, minus the angular span of the key engaging surface.