Found this in the Air Malta web site:
It seems that travelling with Air Malta, a passenger has to purchase a Sports Baggage voucher and firearms/airguns fall under this catagory. It's Euro15 each way making it Euro30 in total so it's cheaper than Easyjet. Maximum weight is 32kg and vouchers must be bought 24hrs prior to departure.
If travelling with Air Malta it's best to notify them before hand that the Sports Baggage voucher is to be used to carry airguns as I don't know if a police officer is needed to escort the guns on the aircraft departing from the UK. All I know is that departing from Malta this definetaly has to be done, but that will be taken care of from our side.