After shooting this many times, the key is not to pull the rubber bands as far back as you can. You only need a small amount of force to shoot the sinker, so if it mis-fires, try backing off the tension on the bands.
Also, another key part to make sure you get the most out of your shot is to make sure the line spools out of the reel cleanly and easily. I've had other reels and line where if there is the smallest snag, it will limit your distance.
Here is my design.
It's quite simple.
A slingshot from Wal-Mart or Amazon
A decent reel
2 small hose clamps
25lbs tess yellow fishing line
1 large egg shaped sinker
I can launch the sinker around 100 feet in the air with some great accuracy.
Here are some of the details.
25lbs Tess Line