I decided to find out what is going on. So I rented a thermal camera. Turns out heat is the key! Rubber stores the energy thermally. So the bands warm up very much when you draw them out (about 15 centigrade difference, in a fraction of a second). They cool down just as quickly when you release.
Wikipedia confirms these findings. It is an entropy thing. A basic law of physics says that if you bring order into the chaos, heat accumulates, and when things swing back to chaos, things cool down. That is what happens, by pulling out the bands you align the rubber molecules, moving them out of their chaotic relaxed condition.
The scientific information I gathered using the thermal heat camera lead to more experiments: What happens if you heat a stretched band, one that already cooled down to room temperature?
It regains the power it lost during the cooldown phase. That is right, this video has the proof.
Now I had to use this critical information in order to build a slingshot crossbow that can heat the stretched rubber with the
flick of a switch. The result looks bizarre and very uncenventional, but it does work very well.
Three heating films heat up the interior of an encapsulated band box. Using a strong 3300 mAh rechargeable battery, the bands can make the bands about 20 centigrade warmer than the environment, adding (or, better, regaining) about 40%
This weapon can be left cocked for a long time, and then - minutes before action is expected - be brought to top performance.
A nice side effect: The compartment also works as a silencer, it traps the sound as well as the heat. Very important for effective Anti Zombie sniper jobs!
Two new slingshots are also presented as a bonus.