Random location. If the hit roll is a percentile system () then an easy way to work it out is to swap the results numbers and look at a chart which states where you hit randomly depending on the numbers you get. So if you roll a 54 and hit, swap them around to 45, which could be torso for example. Is it? I see it as very simple and relatively easy to remember. There are no mathematics to work out if a headshot does 2x damage or that kind of thing. Also, random hit boxes makes combat even more deadlier. You might survive a shot to the arm, but it means you'll end up being crippled easier.