unmistakeable
For you AK-47 fans; the "crack" is not what I would call loud, that is intensity, but it is distinctly attention getting! And from the AK, unique, I might add. I have heard up to 12.7mm from the wrong end, and I knew immediately the enemy's gun. It has a shock wave with it, and you can sense the damage it will inflict.
I suppose you know, but the boom, boom, boom comes a moment later, unless you are unfortunate to be close up to one, and the time delay between the crack and the gun report can be used to estimate what the range is to the shooter.
Some of the replies are referring to muzzle blast. The near passing of a bullet intended for you is a deadly "crick, crick, crick," as it sounded to me.
Not much acoustical energy; decibels, but it get's your attention.
I know there is a lot to admire about the AK, but I hate them.
It may be irrational, but I can't dismiss it.
No one ever needs to shout "hit the dirt."