Are single cr123a powered flashlights relatively safe
Yes
If you’re talking about heat, then the answer is no. A high-output single cell CR123A light can both burn you and start a fire if things go wrong and conditions are right.
Hi what would you say is hight output for one and a rcr123a would better i assume
Yes, quite safe.
CR123A batteries aren’t capable of pushing high currents either, so in case of a short circuit nothing too bad will happen either.
CR123A is an ideal battery for lights you don’t use often.
They keep their charge for 10 years or so.
Do high output CR123A batteries even exist? (if so, i’ll edit my previous post)
Or do you mean *R*CR123A (better known as 16340)?
I could be mis-remembering, but it seems like there are CR123A lights with little or no low voltage protection, because they are designed to get everything they can from non-rechargeable cells. If you use a rechargeable RCR123A (16340) in those, you could over-discharge and then have a fire hazard when charging.
But the terms are too generic and common for me to be able to quickly search and find some sources to back my feeble memory. So I could be wrong.
Use a protected 16340
CR123A and 16340 are very different batteries.
CR123A has a 3 Volt voltage without load, which will sag (drop) considerably under load, due to their high internal resistance.
So if you want to us it to power an LED up to, let’s say 2 Watts, the voltage will drop below the needed voltage for the LED to draw the needed 0.7 Ampere.
So you need a boost driver for a single CR123A light.
A 16340 is like other Li-Ion batteries, it ranges from 4.2 down to 3 Volts, and it doesn’t need a boost driver to push current through the LED, because it has much lower internal resistance and thus much less voltage sag.
They can push more current too.
Maybe this is why some CR123A / 16340 lights are relatively expensive, because you need a boost driver for a CR123A and a buck or linear driver for a 16340, so they put in a complicated buck/boost or linear/boost driver, which can do both.
Low Voltage Protection is a bit of a problem though, because a CR123A will definitely drop under 2.5 Volts (which would be the absolute minimum to safely use a 16340) under maximum load and towards the end of it’s life (charge).
I guess that’s why many of those lights come with a protected 16340, often with USB charging port.
The charging / protection board also keeps recharge current lower than a regular 18650 charger would.
Well, in the olden days, lots of high-end CR123A lights used a 3A configuration, for example Surefire's high-output lamp assemblies used in their military series and other lights. This will give you a lot of heat. You needed gloves when running them for more than a couple of minutes.
But I think the most important issue is what happens when something goes wrong and the battery is shorted. I had one of the tiny Olights that did this right out of the box. When you put the battery in and screwed on the tailcap, it became very hot within a couple of seconds. Electronic switches are not as safe as mechanical ones, they could fail at any time, in your parked car, in your backpack, in your sock drawer and so on, and a shorted CR123A does not just generate heat, it also has the potential to generate high-pressure gas. They have a safety vent and will leak hydrogen. In a well-sealed flashlight body, the pressure will build up. Tailcaps from flashlights supposedly went through a car door, wood drawer and other stuff in incidents widely discussed on another flashlight forum.
Another issue is physical damage to the battery, but I'm not all that concerned about that. Here's a video of it anyway. It gives you an idea about what could potentially happen when a CR123A is in thermal meltdown. https://www.youtube.com/watch?v=fG_UuPmLO1c
But those Surefires use 2x CR123A in series, don’t they?
I don’t think you can successfully draw 3 Amperes from a CR123A, but with 2 in series you can use a buck driver that will significantly reduce the current drawn from the batteries.
They used 1, 2, 3, 4 or 6 batteries, no electronics, direct drive.
Whoa ! 
I fear my ex more than this.
Once CR123a are drained do they become safe