Customs is not the problem.
As far as I am aware it is still illegal to own or posses a air rifle in Austria. In France, if you have more the 6 ftlbs you need an Fac.
So even if you get through customs, if you get pulled over by the police the defence of "I am only driving through your country" will not work.
In the Isle of Man handguns are legal, but if you bring one over to the mainland on your way to america " where they are also legal" you will still get nicked.
You have to abide by the laws of the country you are driving through
Somehow I made TEAM ENGLAND 2012 and 2013
Illiterate writer for AirGunner
The real Kung Foo Panda.