Rubberband Gatling Gun
Perhaps its a symptom of the world we live in, but even as a young nerdy kid I would shoot rubber bands off of my fingers. You know the method, form your thumb and forefinger into a gun shape, stretch a rubber band between the two fingertips, crook your thumb to let 'er fly!
More ambitious kits in shop class would secretly create rubber band guns out of wood. Once they'd crafted a suitably gun-shaped piece of wood, an attached clothespin kept the rubber band cocked. A little thumb pressure at just the right moment and THWAP! A silent attack leaving behind no forensic evidence except a common little rubber band.
Those kids in shop class have since grown up, and have escalated the elastic band arms race to see who can concoct the most devastating implement of childhood warfare. One of the more elaborate examples is the above gun from Backyard Artillery. This baby can fling 144 rubber bands (yes, that's a gross of stinging rubber ammo!) without reloading. Of course power comes at a premium, and only you can decide whether $395 is too much to pay to protect your garden shed.
If you're not scatter-gunning down hordes of invaders, perhaps you can make do with a smaller efficiency model. As a Chicago resident and fan of The Untouchables, They have many gun styles, but I'd really like a pearl-handled Derringer for my moll to keep tucked in her garter.
Oh, and you can also pick one of these carnival-styled targets as a safe alternative to shooting other people. I'd certainly never shoot at a person. Never, never. Hey James, is that the sunburned nape of your neck? TWANG!