30 round aluminum magazine loaded with subsonic rounds, intended for silenced firearms. Fits most STANAG type rifles, including AR-15 designs (M4 and M16) as well as the L85A2. Also found loaded with supersonic rounds for use in regular weapons.
If a non-silenced weapon is loaded with SD mags, zombies will react to the shots as though they were silent. However, they will sound normal to players and the rounds have reduced range and accuracy.