Hey, Dan, you just triggered a hazy memory. I seem to recall reading about a self cocking mechanism for a cross bow somewhere, I think it used a small electric motor. Not that big a leap from string to spring, I'd think. It would have to be a fixed barrel, but maybe put the motor in the butt, with a linkage to the spring shoe. Your'e a tinkerer, there's the basics, go for it! All development costs are up to you, but if it turns out to have a market, I'll expect a 50% cut, what do you think, sound fair ;-)? Of course, thinking that hard almost caused me to break a sweat, so maybe it should be 60%! Seriously, this might be worth playing with. You might want to look at some of the robotics parts sites for more inspiration. Have fun.
Dave