10.3mm works out as .406 so this could be another of those dreaded "un-necessary" new .400's/.411's.

I assume it is based on the 8x68S case?

Bullets seem light for calibre at 13.5 gms / 205 grs and 18.5 gms / 287 grs. No mention of ballistics yet. Pretty poor they don't mention the velocity, even the usual exagerated Euro velocities they usually quote.

Or what existing rifles are chambered for it?

Now has Daryl in the past made one of these rifles and reloaded for it?