Sigma8 calibrated target
Print it, shoot it, and Qual8 does the rest.
The Sigma8 Calibrated Targets use printed ChArUco patterns that let computer vision square your photo from any angle and accurately map every bullet hole for you. The patterns calibrate the size of each pixel in the image, so every measurement is true to scale.
If you want the highest precision, shoot Sigma8 targets. Download them for free and print at home on heavy 60lb paper for clean bullet holes.