As far as i can tell, a choo choo gear can only have an axle on one side, since the linkage needs to fold past the center of the gear on the other. I would definetly use HS axles, but other than that, how does one support this gear so that nothing is bending and stressing? It seems like the amount of force a launcher has would break something with such little support.
Radial release mechanisms are definitely a challenge with plastic gears. One of our teams experimented with one in early competition. A couple tips: the HS shaft should be as long as possible to give a better lever-arm for the cantilever (support it near the radial release gear, and again near the end of the shaft). The large gear used for the radial release should be bolted to a back-up gear, and it wouldn’t hurt to use a couple high-strength lock bars to reinforce the gears (shameless promotion: HS lock bars are available on ebay, amazon.com, robosource.net, or direct from VEX Team Virus).
@kmmohn Thanks for the tips. We were considering basically splitting the mechanism into two sides to reduce load (basically two choo choos). Have you worked with them enough to know if this would make a reasonable decrease in part wear? It would be a lot harder to fit on our robot, but I dont have enough experience with them to know if a single one can take a large load or not.
This is what the team had done, making a mirror image. They thought about connecting the two sides with a geared shaft, but never got around to it. It is prone to breakdown, so be sure to build it in a modular fashion.
Splitting the mechanism is a great way of reducing strain on the axles, not to mention the entire structure of your robot. We used this method and had a really reliable catapult. (It only broke once, and that was a stall that we are pretty sure was caused by a bad battery.) Keep in mind you need to keep the width of the system to a minimum. To do this we didn’t use collars and used 1x bar for the lower struts. The friction of the force pulling on the gears alone was enough to hold everything together.
Here’s a photo of what we built, this was during assembly so things weren’t quite lined up perfectly yet. 
We did two things–1) put the gear on the horizontal plane vs. the vertical and 2) used a 2" screw vs. a shaft. I think also it is important that the screw or shaft be supported at two points along its length before it is cantilevered.