Can you assume that a subsonic bullet has a uniform shock wave as it passes?
If so, microphones should be able to detect that wave. I would first look at the waveform out of two identical microphones on a dual-trace oscilloscope as a bullet passes about 6 inches from each mike. Then fire a shot much closer to one microphone than the other, and see if you have a reasonable signal difference to work with..
CAUTION: "Terry is full of Stories" WARNING:
Years ago, the professional sharpshooter at Winchesters in New Haven named Jack Lacey shot a Necco candy wafer out of my fingers at 50 feet. Wow. About 60 years ago.. I THINK I could hear the bullet pass before the sound of the gun. Very possibly my young imagination.
Needless to say, that probably doesn't happen any more. I was captain of the Rifle Team and I used to drive with my model 52 Winchester in the back seat and carry it into high school.
Ancient History 8)