I've only read about these in the US mags but never seen one in person or even heard about anyone that does, would be neat to shoot and try out for sure. Kind of goes the opposite way of the magnum craze these days which for one makes me happy.
Think at least .45ACP, or there abouts. It's just under sub-sonic, so there is no crack, but you still get the boom from the powder. Check out U-Tube, as there used to be a vid or two on there.