Another collector I know acquired two of these. They are made of turned steel plated with brass and copper. It look as if the “bullet” and “case” were made separately and then crimped together. One appears to have lost its plating, presumably where someone has tried to polish it. Are they from one of the Japanese model guns? They appear to de dimensionally and visually similar (but intentionally different from) 5.56 Nato.
They are 5.14mm (.203") “Calibre”, with a “case” length of 36.11mm (1.422").