A guy I went to OCS with was in an Air Defense Artillery unit in Vietnam. He said the only time he was under fire was when the VC hit his base one night, and that he fired his M16 blind from the top of the berm. An actual percentage of the number of guys who might do that is hard to say, just like S.L.A. Marshall's assertions about the number of men who didn't fire their rifles. When non-Infantry types suddenly find themselves unexpectedly in combat odd things happen, like Jessica Lynch's Ordnance unit in 2003.