Definately a .22 by the absence of the gas block and return spring tube. Also the cut-off pistol grip thinggy. I would say that it was to replicate the or fit into the M8C rifle mount on top of the wombat. It wouldn't sit inside the main armament barrel as the mag would foul the chamber.

All this sub-calibre training and electronic laser simulation - they were just starting to do simulation training with hand grenades too - is really no substitute for the real McCoy