WCC on the headstamp stands for Western Cartridge Company, so it's an American cartridge.
From your description, it sounds as though a standard 45-70 round has been necked down.
Apart from that, I can find no reference to such a caliber being used in the war.
Possibly, it is a commercial round, altered to give a higher muzzle velocity.
Some snipers experimented with commercially available rifles, rather than standard military issue, trying different cartridge sizes, to effect a better hitting range.
If you can find out any more info, even when and where, the weapon was used, we might be able to find out more about it.
Hope this helps a little.
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