The strange thing about sonic booms, and the objects travelling so fast that they create them, is that by the time you have heard the boom in question the object will have already passed you by. Such is the nature of going faster than the speed of sound.
It is that phenomena that leads to the saying of the snipers shot. Put simply, it says that if you are ever being targeted by a sniper, you should not really fear the crack of their shots. If you are alive long enough to hear that bang then you should let out a sigh of relief as that shot was not meant for you, or the sniper in question is simply a poor marksmen and is currently readjusting his aim, in which case you should probably stop sighing and start running.
Put even more simply, the shot that will be your ultimate doom, is the one shot that you will never hear.
