Dummy cartridge i.d

This cartridge has puzzled me for years! I first found these about twenty five years ago whilst I was rummaging through tons of ammunition backloaded for disposal in a British military EOD bunker. I’ve found several identical specimens since then but have always thrown them back onto the heap having concluded that they were no more than a rather elaborate dummy cartridge for a replica or toy gun. They certainly don’t resemble any military calibre that I know of. Having found yet another one perhaps this time around I’ll give it a little more thought and I was hoping somebody might be able to tell me what it is. The case is machined from mild steel which is then plated with zinc, cadmium or something which is similarly shiney. It is strongly magnetic. The bullet appears to be rivetted to the case and whilst strongly secured is able to revolve freely. It is also strongly magnetic and I’d guess is copper-plated. There is virtually no extractor groove or rim which surely would be a necessity even in a replica weapon. Any ideas at all please?

Dimensions would be helpfull.

Apologies for the oversight.
The approximate dimensions taken with a ruler are;
overall length 50.5mm
caselength 36mm
base width 9.5mm
calibre 5mm

Looks to me as a toy.