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I experimented with these a few years ago. Didn't really find a benefit other than that air resistance may help them stay spin stable, which is potentially useful if using very long-aspect ratio projectiles.

The concern I have with this is that the grooves (or fins or whatever) that are carved into stone will need create enough lift (drag) to change the stone's direction and spin which was applied by the sling.  Unless of course if the throw from the sling (meaning the projectile orientation and spin) was perfectly aligned with the grooves carved into it.  Because stones are much smaller and denser than an arrow, a lot of air will need to be caught to adjust the stone's flight.  I don't think there would be much effect unless whatever it is that's catching the air is very substantial.

Just my thoughts.  I certainly don't know.  It will be interesting to see a test.

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This is fascinating and exceptional! Have you done any long shots?

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