So, first off i'd like to apologize for the random cross-hatching going on in my attached image.
Here's another solution on supporting a shooter wheel to be direct driven by a MiniCIM, as well as a breakdown on the factors that were considered when this was designed.
I'll be referencing this image.
First, note that we decided to support the wheel from both sides of the shaft. The bushings inside of a CIM and miniCIM are NOT designed for axial or radial loads, and will cause early failure of the motor if they're subjected to too much force.
Second, our method of clamping the hex shaft onto the CIM shaft is based off of available hardware, as well as keeping it mechanically simple. We're not driving the robot with this wheel, so a split hex with a collar is good enough, especially when a bit of keystock is thrown in the mix.
Finally, the whole system was designed to keep the CIM away from the forces imparted into the shooter when the boulder is released, while maintaining as much speed as possible. Since there's only a single shot allowed at a time, we weren't concerned with creating a heavy flywheel to maintain wheel speed post-shot. What we wound up with was a compact and fairly simple method of propelling a wheel.
The bits I'm not showing in this drawing involve shoving the ball very precisely into this wheel and a second exactly like it.