Rad-steel

Rad-Steel is a type of refined Steel found mostly on Earth. Ammunition made from Rad-Steel is immune to the cordolaine signal utilised by the Sontarans. Steel cased ammunition is historically used on earth primary by the Soviet military and it's affiliated armed forces, it is generally considered to be lower quality compared to brass shell casing and has been mostly phased out of military service since the early 21st century. (TV: The Sontaran Stratagem, The Poison Sky)