This statement, while true, is misleading. The problem is that it assumes information must be obtained via experiment to be valid or useful. We don't need to conduct an experiment to determine that fire is hot, or that being shot is bad for our health. Nor is information gained from a scientific experiment necessarily valid or useful. The problem here is that we are so accustomed to seeing statistical data presented as percentages that we automatically assume all percentages are statistical.
Samples intentionally exclude qualifying events.
Marshall pursued all of the shootings he could. He did not pick and choose from qualifying shootings. His data contains all of the events which met the criteria of one shot to the torso, and to which he had access. Which is not to say that Marshall's information truly represents the effectiveness of all cartridges. The subset of information available to Marshall may, or may not, be representative of all shootings.