A 7-year-old girl was killed and her 8-year-old sister seriously wounded when a neighbor boy dropped a high-powered rifle and the gun discharged, police said. The boy was apparently playing with the gun in an abandoned apartment at 936 S. Albany St. near downtown Los Angeles. The shot ripped through the apartment door and struck the girls, who were playing in a hallway. The older sister was in critical condition after surgery at Childrens Hospital. Police did not release the names of the children, but said that several illegal alien families live in the building. The boy was not charged.