.303 cartridges

".303 British, or 7.7x56mmR, is a .311 inch calibre rifle and machine gun cartridge first developed in Britain as a blackpowder round put into service in December 1888 for the Lee-Metford rifle, later adapted to use smokeless powders. It was the standard British and Commonwealth military cartridge from 1889 until the 1950s when it was replaced by the 7.62x51mm NATO."

John Ousterhout

There is no such thing as a "Springfield .303 cartridge". While Springfield did make rifles that fired British .303 cartridges, the cartridge should not be called a "Springfield .303 cartridge". The cartridge was almost universally called "Brit 303", "British 303", "Enfield 303", or just "303". I would expect it to have been most likely fired from an Enfield, either a No. 1 (the famous "Short Magazine Lee Enfield" or SMLE, pronounced "Smelly"), or a No. 4 (WWII-era issue to British commonwealth forces except Australia, who continued to manufacture their own SMLE's). The extractor marks on the cartridge might offer a clear indication of the firearm that fired it, and the headstamp would identify who made it and when.