It's been a long time since I put a clutch system into any Halo but IIRC leaving on one of the circlips that should have been removed can result in the drive sitting a little too high and that would cause the rubbing. I may not be remembering it right though, it's probably 6+ years since I worked on those projects
The material is an awesome urethane. One of the issues involved with hand molding these is that I couldn't use the normal tongue and groove to completely lock the shells together and so they can flex a little. I have taken hits (and purposely shot a shell at close range) and haven't had any issues.
The other downside is that the hand molding uses flexible tooling which can lead to a little more variations/gaps in the parts than using steel tooling and injection molding. If someone wanted to invest $30,000 I'd have a real set of tools made... lol but until then this was the only way I could find to produce these at anywhere near to a reasonable price.