Don’t know if this will work in Europe, but in the US, a starter gun is considered a weapon. You simply declare you are carrying a weapon at baggage check in, they issue a little label you sign and they stick on the bag, and the bag doesn’t get rifled or checked, because the TSA most definitely doesn’t want to lose weaponry in the airline system.

Pack a Gun to Protect Valuables from Airline Theft or Loss – Security – Lifehacker.