Hi Gunwolf! Thanks for your efforts. Thusfar everything that I have looked into points towards Sweden - when you put it all together.
1. The simplified Swedish crown with cross above which is a very close match to the one on the gun.
2. The stamp "choke" - is exactly the same in Swedish.
3. The stamp "stil" repeated four times means "style" in Swedish.
4. The stamp "ful" is simply an abbreviation of "fullständig" in Swedish and commonly used that way.

Questions remain:
- The 12C inside diamond - I thought this was exclusively an old Birmingham proof mark...
- The juxtaposed letters that appear to be T and M under the crown...