Xtar VC8 accepts extra long usb protected Imalent 21700 cells (modified by shaving the white plastic around the anode), and big solder blob 40T works great, after clean up tube-head connection and just barely tool-tightened. Hand tight is just barely ok after clean up the thread glue. Slightly tool tight is the deal. I got two of these, all good. Similar size class pocket rockets are more throw, this beam is all wide, very bright close in.