They shot a plate that lifted a weight. In effect converting kinetic energy into mechanical energy. They knew how much the weight weighed and they measured how far it was liffted. Then, using 1/2 x mass x velocity-squared equation they could back into the velocity of the shot.
Clear as mud I am sure. I have seen pictures of a machine and it looked something like a saw horse with a plate hanging down and a tape measure attached to it. When the shot hit the plate it would raise it and the tape measure would record how much.
Best,
Mike
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