With some flashlights that take two CR123 primaries, you have to bore out the inner walls of the body to get an 18650 to fit inside. (Especially the case with SureFire flashlights.) Though a 17670 would fit without having to bore out a light. Which would be a good alternative since boring out the inner walls of a flashlight is not as easy as I made it sound.Blerv wrote:It's a bit of a can of worms on that question :) . Best answer is to check the manufacturer specs on each light. The 18650 is approximately twice as long as a CR123 and 2mm wider in diameter (18 vs 16). Usually a light that will take two cr123's will take a single 18650. It you want to go rechargeable power it's a great cell; massive capacity and high voltage. With safe charging practices it's also safer than two CR123's because there is no chance of cross-balancing.
Really though, it's mainly a perk if you want runtime and high output in a small package. A few AA's can get you 1000 lumens these days and the only thing you give up is a bit of width or length.
Though such cells can be potentially dangerous. Li-Ions are not as inherently safe as NiHM cells. It's best to avoid 18650s and other Li-Ion rechargeables until one is fully knowledgeable about the dangers of such cells, and how to reduce that danger so that the lights which use such cells don't violently vent with flame (basically, explode).
While CR123 cells can also vent with flame, following a handful of basic rules will greatly diminish their potential of venting with flame. Cross-balancing isn't an issue at all. Never mix & match cells. (A good volt-meter helps with that. And isn't prohibitively expensive.) Never mix & match old and new cells in the same light. Never mix & match cells from two different brands. Never mix & match cells that have been used to illuminate, with brand new cells that haven't been used yet.
The last two rules are:
1) Only buy Made in America or Made in Japan cells. No-Name, Made in China, CR123 cells are dangerous junk! Avoid at all costs! (No matter how temptingly low the asking-price might be.) They're just not worth it. If you want to save money on CR123 cells....
2) Buy online from a trusted online shop (Such as Lighthound, Bright Guy, or Optics Planet). Buy in bulk. And once again, only Made in America or Made in Japan cells.
My last bulk purchase was a Case (12) of Energizer CR123 cells from Lighthound. Cost was $16 and a few cents. Compare that to $9 for just one Energizer CR123 cell at any Brick & Mortar pharmacy store. What I spent on those 12 cells online would not even be enough to buy two CR123s from a physical store.