This project is for helping people in rescues: a home-made line gun capable of launching a flotable projectile to a man having problems in the water (Right now, In Colombia we have an winter emergency with storms, floods and avalanches, so I think this could be an useful idea for helping rescuers in his job). It could be useful to pass a line from one side to another, too. Or for grab the nearest tree or structure.
How it works? Like the weapon of an action figure. You insert the projectile, the spring compress, and the back part of the projectile gets caught in the trigger. When you press the trigger, the back part is released and the projectile is launched.
NOTE: because the position of the camera (and my bad aim), it looks like a short distance. But the steel cable is 9 meters (30 ft) long, enough for catch a tree branch if you are dragged by a river. (The next week I will do the experiment with a longest cable)
Without the steel cable, the projectile can fly about 20 meters (66 ft).
And why a spring? It's "easy" to recharge and you can do it over and over and over again. Air power could work, but if you want to recharge it, try to find an air compressor in a disaster area or in the middle of the forest. Gunpowder is another option, but imagine you are dragged by a river (powder and water are a bad combination) or inside of a burning building (powder and fire are a good combination, but very inconvenient, too).
This instructable is not the whole idea, but a concept. It's just the begining. Instructables.com is the home of the smartest minds of the world, and it would be great if this become an open project. Somebody carry on making a better spring launcher, and then another one improves the line system, and a third one implement a more resistant cord, and so on. Who knows? maybe one day every one of us could have a grappling gun in his emergency kit. Watch out, Batman!