18650 is a little bigger than CR123's (18mm vs 16.5mm). Modders used to bore out P60 hosts to allow the 18650 to fit and allow the use of a custom head that can be driven a lot harder (current discharge rate) than what CR123's can handle, making them more capable/brighter just by battery design.
a 18650 is @ half the voltage of 2 CR123's. so the head (driver) would have to be designed to accept both.
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