Cordolaine

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