The Ruger 204 cartridge is nothing more than the 222 Remington magnum necked down to 204 cal (5mm) with the shoulder moved forward, case length is 47mm. It has been reported to be very accurate and the AR-15 series are offered in 204 Ruger by some custom builders.

I should also include the 4,85X49mm Britishicon militaryAttachment 120682 cartridge too, also a 5mm bullet on a longer 223 case.

Photo shows 4,85 tracer, 224 Winchester and 222 Rem Special