I have just been given this unmarked .303 Mk VII Ball cartridge and the person who gave it to me suggested that it was made by Frankford Arsenal for the UK in 1941. He also said that this identification was made by Tony Edwards. I was sceptical about this identification and went through my copies of HWS and Labbett & Mead’s ‘.303 inch’ but I did not find anything to suggest that Frankford Arsenal ever made this calibre for the UK or anybody else. The bullet weighs 173.4gn, is of gilding metal and is non-magnetic. It is 29.41mm long.
The primer is distinctly domed and there is a machined groove above the extractor rim which I understand is a feature found on US rimmed cartridges.
I also found this link in the archives in which Tony suggests that he has the same cartridge. He goes on to say that he was told by Bill Woodin that Frankford had indeed made a trial run of .303 in 1940 with a view to supplying it to the UK.