Here it is. I measured the diameter with the same ruler so not the most accurate reading in the world. Looks like 26.5mm to me but I can't find anything other than very modern "vulcan" type stuff and I know this isn't that. I've had it for over 30 years and it was just in my Grandfathers junk box. Looks just like a Japaneseicon 25mm anti-tank round. Couldn't find anything else similar. But as i said, seems to be too large.