I have one of the well-driven TrustFire TR-1200 XR-E flashlights from DX. I have the model that has the extension tubes to allow 2x or 3x 18650 cells. This light has the main body tube (slightly too long for a single 18650) a second shorter tube for use with 2x 18650s, and a third tube to allow the third cell to be used. I was checking different configurations when I noted that with the extension tubes removed, 3x CR123a cells could be used making a stubby TR1200. The voltage is correct as the light is already designed for 3x lithium cells anyway. I don't plan on using it in this configuration as I have plenty of 18650 cells and don't want to deal with the severely reduced runtime of the lower-capacity cells. I just thought it was an interesting observation. Maybe in an emergency when I have no capacity to recharge my 18650s, it would give me an option to use an emergency stash of primary cells on low mode to get light...
I also have an early model TR1200 (before they screwed up the driver). Its still one of my favorite oldies for a flamethrower and few budget lights that run in full regulation when using 3 x 18650's.
Since it will take cr123's you should definitely try out these 18350 IMR cells.
Three of those will net you the same voltage as 3x 18650's but in half the space, And since they are IMR's they will get an extra boost in amperage and is more powerful than the 18650 setup. They just wont last as long as they are rated at 800mAh but ive tested them too 1000mAh and with 3 cells in it and runtimes should still be decent.
What's the difference between the 18350 and a 16340 cell?
I would have some concerns about using rechargeable cells. The fit with standard CR123a cells is very tight, so I would not expect protected cells to fit. Given the vastly reduced runtime due to the lower capacity cells, I would be concerned about over-discharging the cells. Should I be concerned if I ever chose to use this light in the short configuration?
2mm in diameter and 1mm in length.....
18350 IMR's are unprotected as they are considered "safer" and dont need it.
But if 16340's are tight they may not fit. But you get 4.2v on a full charged one and the capacity seems to be somewhat misleading.