Use 2x18350 or 16340 IMR's instead of 16340 LiCoO2. Except for expensive AW cells, most protected 16340 LiCoO2 weren't made to handle the current needed to start up a cold Xenon bulb. Xenon bulbs draw a lot more current than normal when starting up, and will trip the protection on most 16340. The light can still work, you'll just have to flash the bulb a few times to get it up to operating temp, but it can become a real PITA to use. It's not so bad with a forward clicky, but not worth the trouble with a reverse clicky.
I can see that being relatively safe with some high quality 18650 cells. A good pair of Pana or Sanyo cells are consistent enough where it probably isn't a serious issue.
But smaller cells like 16340 are much less consistent cell to cell, and thus much riskier to use. I do not recommend using smaller LiCoO2 cells in series in any situation.
This is especially true with a Xenon bulb. Some of the 9V Xenon bulbs from DX don't look significantly dimmer at 6V than they do with fresh cells. It can be hard to tell what charge state your cells are in unless you take them out and measure their voltage with every use. Why do that when you can get some IMR cells for not much more.
In what light? I have a X6 with 3 NCR18650, a STL-V2 with 2 NCR18650, a L2P with 2 18350s, a TK35 with 2 UR18650FM, a DRY with 3 NCR18650A, a 3+ amp triple XM-L with 3 NCR18650.
It actually is not as dangerous as some make it out to me, I had one incident where common sense says there would have been a reverse charge, but there wasn't. (3 batteries: 2 at 3.6 volts and 1 at 0 volts)
I disagree, I mean I still stand by my statement that 2 LiCo cells in series are safe; whether big or small. I regularly run batteries down to 3v resting, and it is fine. No fires, no explosion, nothing. Because I use good matched cells, when they are the same capacity it is fine.
If you had been slower to post and thought through your response a little more, maybe you would have included the part about, "Because I use good matched cells, when they are the same capacity it is fine" earlier. When it comes to LiCoO2 safety, using good, matched cells isn't a minor detail, it's pretty major, and you didn't mention that anywhere earlier in this thread.
By-the-way, in my experience, it's almost impossible to find unprotected 16340 LiCoO2 that are well matched.
I didn't say it simply because to me that is common sense, and to anyone who uses li-ions it should be. To me as a "pretty major" detail, it is something everyone should know. If they don't know that they shouldn't be using li-ions.
If you want well matched 16340s that are unprotected, these did well in my lights. IIRC, one was 552 mah and the other was 535 mah. Pretty close and IMO safe. I would see no problem in running those down until 0. (but not recharging for safety reasons)