Ejection of an empty clip created a distinctive metallic "pinging" sound. In World War II, it was rumored that German and Japanese infantry were making use of this noise in combat to alert them to an empty M1 rifle in order to catch their American enemies with an unloaded rifle. It was reported that the U.S. Army's Aberdeen Proving Ground began experiments with clips made of various plastics in order to soften the sound, though no improved clips were ever adopted. However, this claim regarding the risks of a pinging empty clip is questionable due to hearsay produced as fact by the only known source, the otherwise fairly reliable author Roy F. Dunlap in "Ordnance Went Up Front" in 1948. According to former German soldiers, the sound was inaudible during engagements and not particularly useful when heard, as other squad members might have been nearby ready to fire. Due to the often intense deafening noise of combat and gunfire it is highly unlikely any U.S. servicemen were killed as a result of the ping noise; however some soldiers still took the issue very seriously. Some US Veterans recalling combat in Europe are convinced that German soldiers did respond to the ejection ping and would throw an empty clip down to simulate the sound so the enemy would expose themselves.