A slingshot is deceptively simple. Just a quick glance seems to be enough to grasp its concept, and this might be part of the fascination we feel when we hold a catapult in the hand and slowly draw back the bands.
However, a slingshot has a large number of parameters that affect its accuracy, shooting behaviour, velocity and energy. Shooters have different demands: Some are hunters, others are long-distance target shooters, some have only access to pebbles as ammo, and so on. Choosing the right slingshot for your task is the first step in becoming a successful shooter.
Accuracy, Velocity and Power
The holy grail for every serious shooter is a high level of accuracy. It is mainly achieved by a lot of training, but the slingshot can help (or hinder) the shooter. If the draw weight is too high, your arms start to shake and accuracy is lost. A high draw weight also fatigues you faster, so you'll fire less shots per practice session- and this will, over time, make you a poorer shot.
However, the equation "less draw weight = higher accuracy" doesn't work either. A low draw weight is less forgiving with errors made by the release. Find a draw weight that works well for you - and be led by shooting results, not by your ego!
If you shoot long distances - and 20 meters are already a long distance for a slingshot- you're much better off with a higher velocity. It keeps the trajectory curve flat, which tackles the problems with wind, velocity variations from shot to shot, and also the angular error, as your projectile travels a shorter distance.
A hunter requires a powerful slingshot - and many non-hunter want one, for the same reason why some people like racing bikes or swinging a splitting maul or having a 5000 Watt bass tube in the trunk. Other shooters deliberately want a slingshot with low power, as this reduces the risk of accidents, especially if the shooter is an infant or uses the slingshot indoors, where every miss can punch a hole in the furniture. "Power" is not the same as velocity or kinetic energy!
Performance and its costs
A slingshot's performance is its velocity / power output, compared to its drawing weight. A high performance is very desirable - it gives you a flatter trajectory curve and more energy while keeping the drawing weight low. However, it comes at a cost: Lower band life, heavier (more expensive) ammunition and bulky forks are some possible drawbacks that come with a high-performance slingshot. Thus, every slingshot is a compromise. You must match the slingshot to your shooting needs, by finding the right balance between band life, draw weight, desired power and velocity.
Source; Melchior Menzel in http://slingshot-shooting.de/