Or use three standard microphones. The positions of them should be carefuly calculated.

Amplify the signal and connect it to the comparator. Then measure the time difference of the sound when the bulet hit the target.

For the resolution of 10mm the time resolution is 29.4us. This time is not hard to measure with standard 8051. So you have 2 time differences measured and with a little more complex geometric reverse calculation, you can get the position of the bulet hit. No DSP or special filtering is required.