Just got a chance to look at one of the news books from the York Archaeological Trust's excellent series, Leather and Leatherworking in Anglo-Scandinavian and Medieval York. Aside from some "crunchy" details about shoes and scabbards in the period, there are some illustrations of some slings or slinglike objects. Heretofore I thought the only sling/slinglike object was out of Hedeby/Haithabu, and here in York we have an array of about 6-10.
Of course, it's ever going to be speculation, since the objects are not fitted with cords, don't show signs of wear, don't have a pile of stones nearby, and are not equipped with owner's manuals.
But slinging is shown in period illustrations.