Proper adjustment of the sling, and the prong over which the sling was looped, was very important in order to achieve the maximum range for the missile. The use of counterweights instead of human power was an important development in the design and use of the trebuchet. It had the effect of increasing the power of the trebuchet as much larger and heavier missiles could be shot from the sling. Also, with the removal of the pulling ropes it was possible to construct a trough under the beam to hold the projectile in the sling. This meant that the sling could be made longer and so have a greatly increased range.
Records exist of trebuchets which used counterweights of lead bars.
These would have been very expensive, so were probably used only for small machines, which required less heavy counterweights. The larger trebuchets would have used counterweights of boxes filled with stones or earth. Records of a 13th century Italian trebuchet show that, with a 1 ton counterweight, the machine could throw a 100-150 kg rock a distance of 150 m. Modern experiments with replica trebuchets have shown that a counterweight of 1 ton can hurl a 150 kg missile up to 180 m with consistent accuracy. These machines added a further improvement to the basic design of the trebuchet.