Jani,
I have shipped antique handguns to Europe and use U.S. Priority Mail, always insured, which is a registered mail product but is expedited and transfers to the destination countries Priority/Express mail system......
I always put the appropriate customs paper work for the U.S. departure customs and the receiving customs paperwork as supplied by the buyer via fax, inside a clear plastic self adhesive pouch on the box......
The Netherlands and Switzerland are especially easy IMO, as their antique firearms laws closely parallel those in the U.S.....as a collector, I am sure you are well aware of your countries laws regarding antique hand gun transportation etc......
All went very smooth and the transition took just over 9 days on the last one.......most of that time consumed in customs on both ends.....
Best,
