interesting cartridge, think I notice this years ago as a possible sporting rifle cartridge dont knowing it is a 600/500 Nitro express rimless

here is more about it https://sites.google.com/site/britmilammo/-5-inch-vickers

"Evolution of the Vickers round from the rimmed, necked down .600 Nitro Express to the initial production .5 Vickers Ball round headstamped "R/|\L 22 469" "

"Eley Brothers were tasked with the design of the new round. They decided to begin by necking the .600 Nitro Express to .5 inch calibre, and initial trials were held with a rimmed case and round nosed bullets. The case was then redesigned with a belt and became the .600/.500 inch, first using the round nosed bullet and then a range of spitzer bullets. Ball, SPG tracer, armour-piercing, Buckingham and RTS/RTT explosive bullets were tested before the war ended. The new round was to be fired in scaled up Vickers and Lewis guns and also in an anti-tank rifle designed by Mr.Godsal at Webley & Scott. (See here)

The rounds were headstamped "ELEY .600/500" and when the case was strengthened slightly and the fireholes modified "ELEY .600/500A".

The Armour-Piercing round had a 570 grain bullet with a propellant charge of 135 grains od Dupont No.16 nitrocellulose to give a velocity of 2,650 feet per second.

Work continued slowly post war, the case was modified again and the belt omitted. By 1922 the case design had been finalised and with a bullet to Design RD 469 this was approved as the .5 inch Ball Mark I."

as a bycatch something about nitro express rifles in the great war https://sites.google.com/site/britmilammo/express-rifles