Projectile launcher theory. ideas?

[Collin Cannon]

Heres a design I put together a year ago and it got figured out to the point i might be able to make a prototype. What are some design flaws you guys might be able to point out, or some advice you think I could use.

 

[Simon Bridge]

The slider will jam due to off-center pulling.
The spring will catch things as it contracts.
The pulley and main support are cumbersome considering what they need to do.