I started doing local Practical Precision Rifle competitions at my nearby range, and have been having a great time. If you aren't aware, practical matches differentiate themselves by having steel targets are various distances that you engage from various positions, with scoring based on both time and accuracy. Up close, its obvious when you've hit the target as you will both see the impact, and hear it quickly. But it gets slightly more difficult once you get out past 500 yards. For that distance, the guys that run the events use sensors which register the impact then light up a LED that you can see through spotting scope. I was shocked to learn they were paying 150 dollars for them. They were pretty basic looking and they were breaking regularly. The theory behind it was simple, just applying the classic Arduino Knock Tutorial. So I put it together in a format that seems to work well as a target impact sensor, and managed to come in at about 50 dollars. The Soldering is extremely simple, and the code is an adaptation of the code from the classic Knock Tutorial.