Pistol rounds travel anywhere from 650fps to 1500+ fps. And rifle rounds travel at 2000 to 3600.

So measuring the time it takes for the bullet to pass one detector, and then the timer would get stopped at another, is going to be tricky. But definitely not impossible.

The target system will most certainly be a consumable. It will be destroyed over time. Especially with rifle rounds. The previous suggestions of video recognition, is a good one. The range I shoot at is an indoor one. It has a bunch of shredded rubber tire as a backstop material. When I shoot at it, there is a noticeable displacement of backstop material. Perhaps this job is not one for the 8051, but rather a Pentium IV, one that scans the bakstop many times per second, looking for some part of the frame, that differs from the one before it, and then localizing the region where the difference occurred.