I've made 2 trips down to Greece
from Stockholm delivering to my boat 2 life jackets at a time. Both trips were with Lufthansa and had no problem.
I tried to find on their site the rules for them without success, so I then looked on the Qantas site (I'm Australian which is why I tried them) and found they had rules saying you can take 2 co2 canisters. I figured that other airlines would be the same and never bothered to ask Lufthansa.
On my first trip I had taken the canister off the jackets and when I unpacked my bag found that they were inside my socks. So at either Stockholm, Frankfurt or Athens (flew domestic) someone did examine them.
The next trip I took the hint and packed accordingly. 4 down 2 more to go