I posted this cartridge here on here some years ago however it was not conclusively identified. It was suggested that it might be a blank of Canadian origin but that was about as far as we got.
However I have now found an image of this same cartridge on Municion.org where it is described as a ‘plastic instructional cartridge exclusively for use by the Royal Military Academy, Sandhurst’. For those that are not aware RMA Sandhurst is where the UK’s army officers are trained. I certainly never came across these cartridges during my time in the British army - however I never served at Sandhurst.
The cartridge is entirely of plastic, including the separate black head, and is unprimed. The primer pocket is very small and may just about accept a pistol-sized primer however I am not certain now whether this is a blank or a dummy cartridge. Does anybody have any thoughts?