My research so far - the spring rates of springs loaded in parallel are additive. With 5x 10-in*lb springs, we can produce 667 newtons of force when pulled back 3 inches (springs in question are 5.5 in long) . That will produce an acceleration of 303 m*s^2 on a 1 lb (2.2 kg) albeit for only a fraction of a second.
http://www.amazon.com/Extension-Spring-5-625-Long-080-count/dp/B001292S08/ref=sr_1_1?ie=UTF8&s=industrial&qid=1257718057&sr=1-1 These are the springs we're looking at. We'll need to use an even number of springs, so either 4 or 6 (or 8 or 10).
So, what do you guys think? Practical? Will we be able to get any distance? Any suggestions?
A ROUGH picture is shown below. === are spring tubes, | are handles, and left arrows indicate direction in which springs will be stretched, and the right arrow is a dart flying out of the center launch tube.