No, most airlines designate A300-600 as AB6. The A340-600 is usually designated as 346. The A300 was the first plane Airbus made (no longer being manufactured) while the -600 model was an update of it (but still a decade older than the A340-600). Unfortunately they are at the opposite ends of the spectrum (346 is the nicest plane Airbus makes, while AB6 is not).