Raimey, you may be right, I have no pictures for the moment - will take some later.
My friend is going to restaurate the gun (not by himself but by Frederik van Breen from The Netherlands:http://www.gunsandgunstocks.com/documents/132.html)

Jani, that is the weapon of the owner's family since more than 700 years and yes, indeed these are Gams horns.

Gunwolf