The only problem with an optics/sensor/etc , is that the optics/sensor/etc have to be infront of the target..
I've done a bit of shootin' , and can't hit the arse end of a bus, so you optics/sensor/etc would be in real danger and need changing frequently...

But then I thought about the jamaican drum's....

Why not produce a tuned steel plate that sits behind the target .When the bullet hits the target it will ring at a frequency dependant on where it hits. Then use a microphone and a PLL to detect exaclty where.