One of my most powerful slingshot crossbows is the winch operated hand cannon. It can handle much stronger rubber than any conventional slingshot. But it has an active draw length of only about 75 cm!
The speed (and energy) of a 25 mm steel ball fired from the weapon is an impressive 45 m/s, about 67 Joules of energy. But I extended the "barrel" to twice the original length, giving the weapon a total acceleration length of 165 cm.
This leads to a speed of 69 m/s, increasing the energy of the 25mm steel ball to about 162 Joules - 2,4 times more!
As a bonus, a new two shot bullpup crossbow is shown. It fires two full size arrows, either at the same time or in consecutive order. Outfitted with a surplus G36 scope, it is really accurate and powerful enough to penetrate a car.
I decided to present a prototype in this week's bonus.