On the "Bulldog" level, after you complete the first part and are able to select your group, choose the antitank gunner. When you get the chance, kill him. Do not try to revive him and wait for him to die completely. Once he dies, his rocket launcher will disappear but you can still pick it up. You will not see anything in your hands and you can still fire the weapon. The rockets will come from wherever you killed the antitank gunner.