You will have to tune it down from from 12
ft.lbs/16
Joule, which I suppose it is now, to the Swedish legal limit <10
Joule, appr. 7,3
ft.lbs.
Preferably you let an airgun workshop do this so they can write a certificate that it is under 10 J.
Then you will have to find a carrier that accepts guns. If you bring it yourself an aircraft carrier will consider it "a gun" and you will have to pay extra for special handling.
If you import it as it is in 12
ft.lbs without having a FAC you will have problems if it's found by customs. The least that will happen is that you will get your gun seized but you also may get charged for gunrunning.