they are not, or at least not doing what you think they are. the RCR123A is a size, RCR stands for rechargeable cylindrical lithium based battery, and 123A is simply 1 X 2/3 of an A sized battery cell (17500). you'll also hear this battery size commonly referred to as a 16340 (and occasionally referred to as a 17340, 17335, etc.), meaning it is roughly 16mm in diameter and 34mm in lenth, with 0 indicating that it is a cylindrical cell.
now, this battery (RCR123A/16340) is available in 3V (really 3.6/3.7V but with a resistive diode), 3.2V (LiFePO4/IFR), and standard 3.6/3.7V (LiCo/ICR, LiMN/IMR) configurations. you can only charge the standard 3.6/3.7V configuration of this battery in the pila, and that is what users are saying when they state that it charges an RCR123A or 16340. the 3V versions need a special charger, even though they are actually 3.6/3.7V, and of course the 3.2V LiFePO4 batteries need a special charger as well or one capable of limiting the current to 3.6V at full charge, and not 4.2V like the pila or other 3.6/3.7V battery charger.
I guess I need to search for 3.7v RCR123A's and then pick the correct atty if I plan on trying the ultramax because Im not buying another charger.
the ultrafire wf-138A is a great solution for charging that size battery in 3.2V or 3.6/3.7V configuration, or you can go with the wf-188 which also has a dual voltage configuration. but going the route of two 3.7V RCR123As in place of LifePO4 or resistored 3V batteries is not a bad idea, and it will extend the usage time because you'll use it with a HR atty which decreases the amperage demanded of the batteries and there's no resistor in there taking up space or wasted energy. so depending on what atty you were going to use initially with a 6V setup, you'll want a HV version in the range of of roughly 4.5-5.5Ω. hope this helps a bit.
