Like others have said, there should be some visible indicator of a bullet striking the floorboard. Did he buy the ammo in bulk? Is it possible that they tracer rounds? If they weren't tracer rounds what type of bullet was it...full metal jacket, soft point, ballistic tip, etc.? Do you know if it was a bolt or a semi auto? it's a semi auto there should be a spent case in the car, or the remnants of one.