My father, who was a WWII combat veteran, once told me that 1 in five soldiers in combat never fired their weapon. I was amazed at the statement, but he insisted it was true.
Your father might be referring to US Army Historian S.L.A. Marshall's famous assertion that 4/5 US Combat infantryman who fought in any given battle did not fire their weapon during the Second World War. This is a contentious assertion to say the least, and one which is difficult to gauge the truth. This claim was used to change how modern infantry training was, and is conducted, as training was changed to address Marshall's primary hypothesis : that people are fundamentally averse to killing, and must be conditioned to kill on command. Needless to say, this study has had some far reaching repercussions in military theory, specifically with the development of "Killology" which is the study of the psychological and physiological effects of killing and combat on the human psyche; and the factors that enable and restrain a combatant's killing of others in these situations. Lt. Col. Dave Grossman, the lead theorist of this area of study, was highly influenced by Marshall's work.
So the question is, is this number given to us by Marshall true? The answer is "we don't know, but the evidence is iffy". Marshall's ability to actually conduct this study in the manner he claimed he did is suspect. Robert Engen, in his criticism of Marshall's work, states:
"He [Marshall] claimed to have interviewed over 600 different companies, though as Roger Spiller pointed out, going by Marshall’s own estimation of the time needed to carry out a group interview that number was impossibly high and would have had him interviewing rifle companies well into the postwar years, which he did not."
Other critics of Marshall's work claim that his data is in fact, invention, essentially accusing him of fabricating data for the purpose of his writings, and have even gone as far as to question his own accounts of his experience fighting in the First World War, with there being some rather damning evidence that he could not have been present at the various battles he claimed he participated in for a variety of verifiable reasons, for instance, his claims to have been in Bantheville during the fighting there, when he was in fact, records show he was taking the West Point Entrance exam that day.
Marshall's data also only points to one conclusion, that in any given action, only 15 percent of soldiers fire their weapon. There are a variety of other reasons besides Marshall's reductive theory (that humans are adverse to killing) , which he simply does not address meaningfully. It is entirely possible that soldiers simply did not have an opportunity to fire their weapon. Depending on the ferocity of the fighting in that area, their might have simply not been any targets to engage. Soldiers may have been observing fire discipline for any variety of reasons. There are a multitude of other points to make about Marshall's work, and I suggest you look at my source if you wish to read more in depth into these questions of the validity to Marshall's work.Sources:
Engen, Robert. "S.L.A. Marshall and the Ratio of Fire History, Interpretation, and the Canadian Experience." Candian Miltary History Journal