Hi Moggy
Dimensions as follows
Outside Dia. 22mm

Inside Dia. 21mm

Length 14mm

Slight inward taper at firing pin end

Outside of cartridge seems to have marks caused by being clamped inside a barrel when fired. It seems to be very well made out of brass (Much better quality than a shot gun cartridge)

Tried to photograph it but it came out blurred. I dont think my cheap and nasty digital camera is up to it.

Thanks