[Mike Schreiber]

http://www.chiefdelphi.com/forums/sh...2&postcount=12

This spreadsheet will be useful to you. You can determine your initial velocity using energy (as others were saying).

.5 * k * x^2 = .5 * m * v^2

I recommend getting your surgical tubing spring constant experimentally by loading your mechanism with weights so m*g = k*x if you know k and you know how far you're pulling it back then you can calculate velocity. Be sure to include the weight of your launching mechanism when calculating velocity, it is being accelerated too.