Bass-heavy rock act Primus will headline the annual Locobazooka festival on Sept. 19. The Boston event is expected to draw around 20,000 people, according to WAAF-FM promotions coordinator Ann-Marie Blood. The radio station sponsors the event, to be held at Green Hill Park in Worcester, Mass. Rock bands Type O Negative, Drain STH, System of a Down, Static-X, Verbena and others also will perform during the festival.