No, 18650 will not physically fit in the holder - it is clearly designed to NOT accept this size battery.Does a 18650 fit in the battery holder? If this is possible, are you able to put the battery holder with 2 18650s back in the light? Then its maybe possible to use this light with 2 18650s if you insulate 2 contacts and connect the batteries with a wire to put them in series.
Given the popularity of 18650, it stands to reason this was a very conscious design choice by 4Sevens (i.e. they could not design a circuit that would accept three parallel 18650s). IN THEORY, if you could jury-rig two 18650s in series (presumably would need to dremel away plastic, block contacts, and re-wire the carrier, etc.) you should be able to get the light to activate. But I can only presume that it would be dangerous on anything other than the lowest output mode, especially on non-IMR cells. If they could have safely driven an SST-90 at over 1200 lumens on only two standard 18650 batteries, I'm sure they would have offered that.
I realize many here are disappointed about lack of 18650 support in the circuit. Given that, I'm actually glad the carrier won't physically take 18650. Otherwise, it would encourage people to try and mod the carrier for 2x18650, which would likely lead to unsafe current drains on Hi. :shakehead Again, I urge everyone to step back and consider what they are playing around with. This light is designed for 6x primary CR123A, with the rechargeable option of RCRs only.
The "real" runtimes are shown in the graphs in the review - they are what I actually measured in my lightbox, at the Med and Hi levels on both 6xRCR and 6xCR123A. The graphs show relative output (on the Y-axis, or vertical scale) over time (on the X-axis, or horizontal scale).TKS, and what about the cr123, how many minutes will be for real??
As you will see, on primary CR123A, the S18 runs at a regulated level for the first ~50 mins or so on Hi, followed by a slow drop-off in output. On Med, you get 3 hours before the drop-off occurs. On protected RCR (which shuts down abruptly as the cells near exhaustion), I got 36 mins to shut down on Hi, and just under 2 hours on Med.