I have a MP15 Sport 2 and a 9mm XDm 4.5 inch. I shot both back to back at the range and I realized the pistol seems LOUDER than the ar15. Also the sound of the 9mm is more like a balloon filling up completely with air and then "Popping" vs the lower "Bang" sound that the AR makes. What causes these differences? Sorry for the dumb newbie question but I was curious.