To use Newton’s 3rd law and elastic force you must maximize your draw length, and second you must maximize the draw force you can personally exert over the draw length. In other words, the slingshot must be designed (for you) such that you are able to pull back on the projectile as far as you can and as hard as you can, before releasing it. This maximizes the elastic energy storedin the rubber bands which translates into the maximum kinetic energy of the projectileupon release, which results in the maximum release speed of the projectile. This in turn results in the greatest impact energy, and therefore damage inflicted on the target when it is struck by the projectile.
