Just as a side note, to put a dampner on things - anyone planing on going this side of the year book refundable/transferable tickets. The current feeing is they will open without restriction late 2022 but not before the august election.
Also, some shops aren't too friendly to foreigners buying rifs, but are ok with parts and accessories. Echigoya and First were friendly.
As others have said, it will be down to the airline, and some like to charge a not insignificant amount pending on what they class it as. Lufthansa currently charge 250 euro each way plus a security fee if they class it as a fire arm.
Cathy pacific are the only ones I've seen that list it as a toy and just a security check, probably due to the popularity of airsoft in HK
edit: after some searching JAL are ok charge £70 for a processing fee. So get a couple of rifs are you're back in profit