A friend is describing this to me so excuse the unanswered parts that I know should be included.
A Belgium rifle dated 1943 (with the v and a star over it) with 10.3 stamped on the barrel, Nitro proofs post 1924.With a rim daimeter of .550 and a base of .500. The bore is .420. He "thinks" he can feel a end of chamber length of 2". And it is a straight walled cartridge.
The * over v is not the date stamp. It is a controller of proof mark. In this case Isidor Maçon, who was a controller from 1929 to 1953.
In 1943 Belgium was an occupied country. I doubt seriously that they produced any sporting arms that year. It is even less likely they produced one in an obsolete chambering in 1943 for the Russian front.
There is no date stamp on the barrels. I have a thought about who produced the barrels, but am not certain. It would help to see the water table and any other marks on this gun. Then at least we could say who made and when.
Pete