Ooops! Sorry about the adapter suggestion. That was a little off topic. FM and Cryos bodies are 18650 compatible. Another option would be to bore out the body you have already, however that will void Surefire's warranty. I did the bore job on my 6PX before the custom bodies were available. If they were available @ the time, i would have bought one instead. As for the tailcaps, the P,C, and Z Surefires will work on the X body.
