Slings greatly improve the range of most types of hurling devices. They generally consist of a pouch that holds the projectile connected to the arm by two lines. One line is attached to the arm and the other end is in a loop over the pin. Thus when the sling swings out accelerating the projectile the sling will eventually rotate far enough that the line that loops over the pin will come off, causing the pouch to open, releasing the projectile.
For information about adjusting the sling and pin, see the tuning page. For information on making adjustable length slings see the design below.
An explanation of how slings work can be found on Ripcord's How slings work page.
There are many great sling designs at Ripcord's slings page and Siege-Engine.com. One very simple and effective design not listed is the duct tape and string design best explained by a picture, see right. Another picture with the slings full and one with the slings empty. Also, you can scale the hand slings on Slinging.org to meet the specifications of your design.
Being able to adjust the length of your sling is very important to the process of tuning your trebuchet. Ideally a sling will be able to both hold its set length without slip during the intense tension created during the throw and be quickly and easily adjusted between launches.
For small to medium trebuchets using rope as the sling material the Taught Line or Rolling Hitch can be used. Both these knots will hold reasonably well but in newer synthetic ropes may slip during the launch. Tape can be used to prevent this sliding but then makes it more difficult to adjust later.
Knots do not work well however for machines that use flat webbing. One option is to use a CAM locking mechanism found on many lighter weight cargo straps. These CAM locks hold fairly reliably during launches and a very easy to adjust. They do cause abrasion wear on your strap which can weaken it. The higher loads they are subjected to the worse they perform. There is a reason they don't make straps rated over a couple hundred pounds that have this style of adjuster.
Another option for flat strap is to use a doubled back buckle. These buckles are commonly found on climbing and fall restraint harnesses and a typically rated for 5000lbs. Care must be taken to ensure the proper threading through the buckles to prevent the strap from slipping. These buckles are not as easy to adjust as a CAM lock but are still reasonably easy and do not cause abrasion.