"She points to her back, where a knot in the back of Mary’s sling serves as a holster for the shotgun."
That must be some sling if you can use it as a holster for a shotgun! The meaning in this case, is just a plain old knot, where the fabric is tied together, forming a pouch or pocket. But you'd be hardpressed to put a shotgun in it as most shotguns are about three feet long.
The knot in Mary's sling is tied in such a way as to form a small loop through which a shotgun barrel can be slipped, thereby forming a holster for the weapon.
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