The Virgin Australia website states, in relation to firearms and ammunition:
“Not permitted on flights to, from and through Doha, Qatar.”
It must have something to do with the partnership agreement between the two airlines*, because Qatar does allow the carriage of firearms in checked baggage on its own flights, with prior approval. It seems the only way around the restriction is to book directly with Qatar.
https://www.virginaustralia.com/au/e...ngerous-goods/
*Actually, Qatar's conditions for the carriage of firearms say "Ticket issued only on Qatar Airways marketed and operated flights." That would preclude the Virgin flights.