Using the service Royal Mail advise you to use,only guarntees your parcel gets as far as the x ray machine operater.
Hate to say it mate,but i think you have lost the pistol....I was more fortunite last year,when they had my £550 rifle,that had just got £200 of work done on it sent to Belfast,got it back with an apology.I think the only thing you can hope for,that the firearms expert RM have look at your pistol,pull the co2 canister,and say its okay to return to you.....long shot,but you never know...good luck.
Dont anybody even post a water pistol to Scotland using RM