Finally I´ve got the Arms &Armour journal with the article on cestros by G. M. Hollenback

. Based only on the Polibius text, he present a new hypothesis of the cestros sling, since on the dart there are almost no doubts and both authors, Livy and Polibius agree in its description. Acording to Hollenback, the sling could be a simple thong that was fitted below the fins of the dart, as it is seen in the photo. There is also a loop or band so that the shaft end passes through it and the dart don´t slip out during the whirl (this is common in all reconstructions since it is the only reason to place the fins in the middle of the shaft, sacrificing the stability of the flight).
Although I had tried without success this solution some years ago, I´ve made a new cestros with the proper measurements and with the Hollenback fins special shape, and tried it again. Unfortunately, the results were deficient one more time. Sometimes the design works and others not, that is to say, it is quite uncontrollable since the dart has tendency to clog between the cords or to release erratically due to the strong friction of the retention cord on the fin. This causes that the design of the fin is very critic, needing a very exact curvature, and that the material of the thong and the polish of the fin influences much in the success of the release. In addition, the design works well only for a mid power, but throwing with high power the dart is clogged between the cords or leaves erratic. Another disadvantage is that it works only underhand, and the author notices it. This last fact should have been enough for the author to realize that the design is erroneous, since a expensive missile as the cestros was not only to be used like massive rain of projectiles, that is for which serves the underhand style. On the other hand the Hollenback interpretation of Polibius text is a little forced so that it agrees with his design. I think that if Polibius had wanted to describe this design, he had done it in a different way as how he did it

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I think I find unlikely that the Greeks used this design. They were excellent slingers and very efficient constructors of weapons.