The MP5SD (called Covert Submachine Gun) is seen in both iterations of the Mercenaries series. In World in Flames, it can only be bought from Pirate shops under the "Covert Assault Kit", at $10k per unit. In Mercenaries: Playground of Destruction the South Korean elite ROKA 707th Commando Unit soldiers and South Korean Officers seem to use this weapon the most and is the most frequent way to obtain one. Also during the Chinese mission in which you must kill the Russian officers a crate in front of the Chinese HQ contains a covert submachine gun.