Hi Allan,
The easiest would be to ask them what they have access to.
I suspect they can get Sellier & Belot and PRVI. I don't know if RWS has that caliber available. Norma does not seem to.
They can also get Kynoch I think, but of course that's very expensive, and it might take forever to import.
The .303Br is a "new" caliber in France, as it had been banned from 1939 to a few years ago along with all the other military rounds! There are basically almost no weapons in France in that caliber.
The most common double rifle rounds by far are 7x65R, 8x57JRS and 9.3x74R.
Good luck with your project!
WC-
