I wonder if the pop sound is coming from the hollow quill of the feathers or perhaps the hollow bones being compressed on pellet impact and the release of the captured air passing the speed of sound. Maybe its a sonic boom? Kind of like when you took a plastic drinking straw, pinched both open ends and rolled them around your fingers. Then you would have your buddy flick the remaining exposed section of straw with his finger. The straw would erupt with a loud "pop".
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