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Mersatheory differs from practice in that the radius of the trajectory of the projectile when in contact with the sling does not match the length of the sling.
so there's probably a bell curve about efficiency. but one cannot say it's the sling length that makes the difference in efficiency , actually it's the ability of the thrower to make the radius of the curve before release as big as possible with the sling he uses.
some will be able to do it with longer slings others with shorter because of their anatomy(?) or habits.
PS:
Mersa wrote on Dec 11
th, 2018 at 3:48am:
an extremely large one would have such a large window that the thrower would be more likely to make mistakes .
large window means he is allowed to make more mistakes and hit the target, not that he will do more mistakes.