No, the 8x50R Austrian Mannlicher aka .315 IOF aka .315 BSA is not based on .303 brass. The base diameter of the .303 is .460”, while the 8x50R is .494”. The 8x50R was once chosen as the civilian chambering for their India bound Lee – Speeds because neither cartridges nor components, bullets or cases, could be used for reloading military .303s or vice versa. But the then, 1904, still current 8x50R Mannlicher worked in a Lee – Speed magazine and action with just a slight change to the extractor.
The photo shows, left to right, a .303 British, a 8x50R Austrian Mannlicher, a French 8x50R Lebel (.542” base), a Browning headstamped 8x60S, a RWS 8x60 and a Kynoch made .318 Westley – Richards.