I’m almost sorry for such a question that I feel much show my ignorance but as a new member, forgive me……
I have a number of CR123 flashlights, each taking 2 such batteries…
I see numerous comments saying how 18650 can do a much better job……
I have to assume that in some way one can use one or the other but maybe that is where I go wrong….
Cause the 18650 is shorted than two CR123 batteries so that comparison much be based on using different flashlights in each case….
It is also fatter and so does not fit into the same flashlight.
If all the above is true than I have to redo my thinking but if there is some way that one can use one or the other in some flashlights I sure would like to know….
Thank you to anyone putting me straight….
Steve7545
The discharge of a an 18650 is so much more as compared to a standard cr123. The only reason some lights specs say that 2 cr123 will produce more output is because of their higher voltage
There are some lights that support both 2xcr123 and 18650 and are primarily 18650 due to 18650 being fatter. The advantage of 18650 here is it has more energy, rechargeable and safer to use( due to being 1 cell only) compared to 2x cr123
the 18 part of the 18650 is diameter .18MM across
and the 65 is the length …65mm long
and the O is …Round …O circular battery instead of something like a flat cell phone battery
Depending on the driver it may be brighter with the extra couple volts … or it may just burn off anything above 4.2V or what ever the emitter wants.
there are ways to bore some lights out to accept 18650’s …but like someone just said … is there enough material to bore it ? does the 18650 even make it past the threads of the light .?
the main advantage is having to never buy cr123’s again …plus a bunch of other reasons 
you can also run a 16650 or probably a 17650 but the 17650s are probably all old since no one is making them any more … otoh there are good 16650s out there but they are a little pricey… but may be worth it if you really love the light
first CR123battery is17.0×33.5 ,but18650battery 18*65.0.so the size is difference
Also, welcome to BLF! :party: