Well I have boxed up a set of parts and they are on their way to Gilliebear for a mp5k build. It will be interesting to see how the RCM bolt head works. I will let everyone know what we find out.
I have a MP5K clone that was doing the same thing. FTE and the case would get jammed and crushed. I had to carry a screwdriver and pliers in my range bag to deal with it. I was plagued by this issue for a long time and I finally started doing research on the problem last year. I replaced the extractor spring with a factory rifle extractor spring, which did not resolved the issue. I ended up replacing the extractor with a MP5/MP5K 9mm Extractor Enhanced - USA version from HKParts.net and it resolved the issue. My original extractor was very worn and showed up to .012" difference compared to the new US made extractor when measured with digital calipers. Now my MP5K clone runs like a champ.