16340 means the battery has a size of 16 mm (diameter) x 34 mm (Length) x 0 (Cylindrical shape). It commonly refers to the rechargeable version of Li-ion CR123A battery, or RCR123 battery.
Unfortunately, not every 16340 has the exact size of a CR123A. In fact most 16340s are slightly longer or/and fatter than a regular CR123A. We are talking about the difference of around 1mm here so normally it won't cause any problem. But occasionally when a manufacturer tries to design a ultra compact light and make the battery tube size a tight fit for CR123A, the light might not accept a 16340.
Generally speaking, if a light uses twisty switch (Olight I1, Fenix E15, Eagletac D25C twisty, and quite a few Surefire twisty models ...), it won't accept a 16340 as a replacement for regular CR123A. Almost all lights using clicky switchs, either on tail cap or front, can use 16340 batteries. (There are very few exception such as Eagletac P20C series.) All lights that can use 18650 will accept 2x 16340.
Also, 16340 can be either 3.0V or 3.7V. Most modern LED lights can accept either type. Some lights are even designed to utilize the higher voltage to produce brighter output. e.g. Jetbeam PC10 generates 550 lumens with 3.7V 123A but only 250 lumens with 3.0V 123A. However, most manufacturers will not officially support 3.7V 16340 because of the higher possibility of overheating and lose of low-medium outputs.
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