I always loved lego creations that shoot things, especially crossbows. Unfortunately, the bricks are not so ellastic, so some power source is needed. I decided to use stiff string, as opposed to common ruberbands. Instead, the limbs are moved by steel springs.
Reverse draw crossbow by David_Z1, on Flickr
This is how it looks unloaded:
Crossbow - up1 by David_Z1, on Flickr
and here loaded:
Crossbow - up2 by David_Z1, on Flickr
The limbs are not moving that much, but the draw length is quite long due to the compound system. This has few advantages:
-The peak string velocity is 3x the limb velocity
-Bolt acceleration is smoother
-For the given draw length, limbs can be shorter = stronger and with less inertia.
The springs are stretched a lot even in the idle position. Thanks to this, the string force ramps up from 0.6 kg to about 1 kg, greatly increasing energy as compared to going from zero. Moreover, the construction is stiffer this way. The aim of reverse draw construction was to shorten the limbs and move them to the rear, improving balance. The limb movement is synchonised (two grey beams on the front), and the spring tension is regulated (suprisingly, two identical springs were not-so-identical, so this was mandatory).
Crossbow - trigger mechanism by David_Z1, on Flickr
This is a typical crossbow trigger. After pressing it, the white knob is free to rotate, releasing the string with little friction. The knob can rotate backwards, so that the string can be hooked up by just pulling it over the knob.
Okay, for the performance: bolt velocity 20 m/s (about 66 fps). Not exactly blazing fast, but the ammo is quite heavy and stable in flight. The hit effects are usually like this:
Penetration test by David_Z1, on Flickr
I'd still call it a toy, it is no real danger unless someone is shot in the eye (obviously). Hitting anything beyond 20 m or so is very hard. One can also load lighter ammo such as pencil to reach some 35 m/s, but accuracy becomes terrible.