いろいろ調べてみましたが、エアガンは船便でも送る事ができないようです。
今回はX線検査で危険物と判定されて返送されています。
危険物であるにも関わらず稀に検査を通過する事もあるようですが、「エアガンは海外には送れない」というのが郵便局からの正式な回答ですので、何卒ご理解頂くようお願い致します。
I looked into it, and it seems that we cannot ship an air gun in any way even by surface.
This time, it was sent back because it was found as a dangerous article by X-ray inspection.
I don't say that a dangerous article can never somehow pass the inspection.
However, Japan Post's official response is "air gun cannot be sent overseas", so I would very much appreciate if you could accept the situation.