Two situations. As a kid playing one day on a beach in Puget Sound (WA) i kept hearing ziiiip, ziiiip followed by light smack against the rocky bank behind me. Looking out over the water I could see no one within a quarter mile or more. Finally figured it must be real bullets nonetheless and dived behind a driftwood log. They sounded kind of slow and spent so they may have come from the other side of the bay about 3/4 mile away.
Second experience just as kraig mentioned in the pits at a CMP match pulling targets. Distinct difference in sonic crack and thwack on paper between .223 and .30-06. Safest way for sure to hear incoming!!