Cordolaine

A Cordolaine signal was a jamming signal that excited the copper atoms in bullets and caused their expansion within a gun barrel, rendering Earth-made guns useless against the Sontarans. The Sontarans used the signal in their invasion of Earth. UNIT managed a counter-measure by using rad-steel coated bullets. This made the Cordolaine signal ineffective. (DW: The Sontaran Stratagem/The Poison Sky)