From what I remember you need a licence to own an air rifle over 6 Joules (I think it's 6, I know a 12ftp rifle would be over their limit) and you need to contact the local Rozzers to let them know, your not allowed to hunt with any air rifle what ever it's power, if you go to a gunshop he will hand you a rimmy to do small vermin shooting, I'm not sure about the tranportation of the rifle into France but I'm sure if it's going in the back of a car then you should be ok getting it over there and as for storage I'm sure if you show it to be in a sensible locked cupboard and out of harms way.

Why don't you get your friend in France to ask the local authoroties or a gunshop to see if they can shed some light on the subject.

Cheers

Pete

PS I just remembered my brother telling me a French friend he knows had a small penknife taken from him by French customs and in France everone uses penknives at the table to cut bread and cheese with, so go careful.