Not sure, it is what ever CIP says it must be to pass proof...... .311 maybe

Unlike the the US and other countries without a proof house French mfg's and other CIP countries have specifications that include groove diameter, groove depth, width of lands etc for every caliber that must be met to pass proof.

I know you mentioned early on about a 303 with .308 bores, nice idea but not sure you can get it made in any country with a proof house.