Most sniper rifles are between 2,700 to 2,900 fps (feet per sec). see
http://www.sniper-training-guide.com/training-guide/000292.php.
From my hazy memory; M-16 muzzle velocity is 990 m/s (meters/sec) which using 3.28 conversion factor is 3,247 plus fps. M-16 is by far much faster.
The Speed of sound (or Mach 1) at sea level is 1,116 fps
see
http://www.allstar.fiu.edu/aero/mach.htm.
Maybe the large calibre anti-material rifles (firing the .50 cal round) comes close at 2,912 fps (RAI Model 500).
Suprizingly the AK-47 which fires the larger 7.62mm round is slow (at 710 m/s or 2,328 fps) see
http://www.sovietarmy.com/small_arms/ak-47.html.
All in all, you cannot 'siam' a bullet as it travels 3x faster than sound. You hear a crack (when the bullet flies past you) than a boom (the sound of the expended round), when acting as the butt party (in the old days of manual butt parties) raising and lowering the targets for range practice.
BIC ought to be the same but its too noisy to hear individual rounds (and is too close to GMPG).