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I don't know the smallest diameter that a hollow bullet can be made, but the Flatau design ( ring airfoil )swallows the bow shock-wave and has an incredibly high BC at high velocity. At low velocity the BC drops precipitously.
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Flatau's design was a 'theory" that never worked, and after the military spent a fair amount of money on in, dumped it. The problem is that you cannot have a rotating airfoil "with lift".
There were some soft riot control projectiles made, but they had very short range (40 yds), so they didn't do very well... it is now history, along with the perpetual motion machine.
It is physically impossible to "swallow the bow shock-wave", and the energy is lost when it is generated, whether it is "swallowed' on not.
If it were possible, then all supersonic aircraft would adapt this energy (and fuel) saving feature.
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