I have some update;
I slugged the barrel, measured the rifling twist and made a Cerrosafe cast of the chamber and about 1 cm of the rifling.
I sent the Cerrosafe cast to CH-4d to have it determinated and have a forming die and reloading die set made if not in stock.
It turned out to be the 10x47R caliber which they stock all the dies for, so no long waiting times.
The barrel twist is quite long, 1 twist in 545 mm, so I was wondering about what kind of bullet to be used, did some research and came up with a stubby 200 grain bullet to make sure, it stabilizes at a moderate 350meter/second speed.
The cast shows, there is room in the neck area to make the case about 52 mm long to bring the bullet touching to the rifling, which I will do.
Next thing is waiting untill all the goodies arrive and than slowly start forming etc.

I have a double rifle in 10.5x47R with a 0.439" bore.  I shoot a 250 gr 0.429 bullet that has been patched to 0.439 with a case full of Triple Seven or 22 gr of 4198.