Ask HN: How to detect when a bullet hits a cardboard?

Working on a project to create dynamic moving targets, and I need some practical ideas on how I can detect when a piece of cardboard acting as the target is hit. Ammunition used will most likely be standard 9mm Luger or .223 Remington.

Expected problems:

* The target can be as far as 25 yards.

* It is possible a good shooter can hit or intersect with the same spot multiple times, so counting simple circles would not be sufficient.

* It would be best if we can also detect how close to the bullseye was the shot, but that's not a requirement.