Not only is a Blackpowder gun,,,SUB sonic,,
Your chronograph must be broken.
Speed of sound=1116 FPS
120 grains of 2F with a 315 gr .490 Conical =1345 FPS
...................................485 gr =1234
140 grains of 2F ..........315gr =1471
...................................485gr =1345
They are even faster with Sabots, just a poorer Ballistic Coefficient and Sectional Density....but you get the idea.
But the neighbor could also be using lighter loads.
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