We went to Norway via Ryan Air and were told that if the airbottles were found then the bags were allowed to be confiscated without us knowing and the bottle would be destroyed and we wouldn't find out untill we landed.
We got them there and back with no problems (regs detatched and a note stuck to them explaining that they're empty and safe) but it's a big risk. Guns they're even more strict, bags have been confiscated for having TOY guns in them by ryan air, so we posted those.
If you want to take kit, avoid Ryan Air.