Do not measure the circumference of the sling, as this will give you a finished sling twice the intended size. During the standard manufacturing process, only random slings during a production run are tested for compliance; in order to provide proof testing certificates with your order, every sling must be pull tested at the time of manufacture.. Nylon endless slings are an economical choice because the design allows the user to move the contact area with each use which decreases wear and tear on the sling.