By ELTON C. FAY WASHINGTON, May 16 <AP) Tbe Army wonders why aU ita soldiers don't shoot their weapons in battle. Although in Korea American f()rces have a petential "fire power" far beyond that of any previous conflict, the mystery of the non-shooting soldier still per sists-as it has in two wars. Some experts, among them CoL S. L. A. Marshall, retired Army offtcer and military wri~, claim that in a fire-fight no more than one out of four men uses the rifle, a...