The Army didn't learn about the sling as an aid to marksmanship until the Model 1907 Sling was developed - we still use it today, but only with 'marksmanship' weapons, not 'service' weapons - though a hasty sling can be used when instructed.
Prior to that, the sling was to aid in the carrying of the piece.
Same's true of the myriad of European military slings - they just helped to carry the piece when both hands were needed.
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