Could be using a larger caliber with a sabot style .223.
This would give a much greater muzzle velocity, accuracy and range.
The found .223 cartridge could be a red herring
There wouldn't be any reason to salt the ground with misleading cases. The spent bullet would have groove marking indicating a sabot wasn't used.
Question: Did the authorities ever report that a 5.56/.223 Remington case was found?