TLDR: Outdoor actual use was fantastic with very high sustained output, indoor testing light gets very hot.
Indoor without cooling: Output started @ 3200 lm, then stepped down to 2000 lm after ~7 minutes. External temperature goes up to 57° C and stays there. Handle feels very hot at this temperature. I turned off the test at 18 minutes; not wanting to risk damage.
In contrast, with ambient temp around 15° C, output stayed at ~3000, and handle was slightly hot, but “holdable.”
Brightness, tested with Texas Ace lumen tube and EVE 50PL battery:
Moonlight: not measureable w/ my lumen tube
Levels 1 to 4: 40, 310, 1040, 3230.
Turbo: 6000 lm (edited)
Info thanks to @thefreeman: Buck driver, level 4 is ~10a, Turbo is 20a.
If you want to measure moonlight and have a lux meter, you can get lux readings for moonlight and level 1, and then scale the lumen reading for level 1 according to the ratio of the two lux readings.
Great (and smart) idea. Why didn’t I think of that. 
Moonlight = 2 lm thanks to @QReciprocity42 's suggestion.
BTW, consistent results: 2, 2.2, 2.1, 2.1, using numbers from level 1 to 4.
Wow, that driver has very good mode spacing. The multiplicative factors going from ML and levels 1-4 are 20, 7.8, 3.4, 3.1. In an ideal world this sequence of factors should be decreasing: faster on the low end and slower on the high end.
In comparison, my 5A 6V boost driver has factors 6, 10.5, 4.5 (in the 50% group), which is not great. The lowest two modes are too close to be meaningfully different, and a very useful medium mode is missing/skipped in that 10.5 multiplicative gap.
Could you explain why this is so? TIA
Yes! It might be easiest to explain by example:
Suppose you are in the 1000lm range. A factor of 4x gets you to 4000lm, which lets you see substantially more stuff. We can probably all agree that 1000lm and 4000lm are meaningfully distinct modes good for different purposes.
Now consider the difference between 0.01lm and 0.04lm. There is still the same x4 increase, but both modes really accomplish the same thing: if you can find the keyhole to your room at 0.04lm, you can also do it at 0.01lm, 90% of the time. Both modes are moonlights and not meaningfully distinct.
From this comparison, it should be clear that lower modes should be spaced farther apart than higher modes in order to be usefully distinct.
Makes perfect sense. Very good point thanks.
This level 4 (highest stepped) runtime shows appropriate step-down then recovery of M21K. This is why at the end of my walk at 15° C ambient the light measures 3000 lm. The sustained output is borderline insane for a light with no active cooling. I love it.
Also re-confirms that temperature with actual outdoor use is much better/cooler than indoor runtime testing.
IMHO, the M21K’s largish head helps with the aggressive thermal algorithm (think Anduril with Tmax reset to 60° C). Better heat dissipation. With a smaller light like M21E, it would be interesting to see response.
I held the button for 5 minutes to check Turbo runtime. The good news is that there’s no news, both LEDs and lights did fine.
The step down stops at 2000 lm, about 5a if I read Koef3’s test correctly. Both level 4 and Turbo appear to step down to the same level at ~2000 lm.
Plan to test again to make sure, but so far I have not seen a lower stepdown level (anyone correct me as needed). This explains why M21K has such fantastic sustained brightness. It also means IMHO similar to all high power lights, this one is best reserved for experienced enthusiast looking for a hot rod racer. Special because it has IMHO the industry’s most beautiful warm CCT and tint in a high-power LED . All that is required is an ounce of common sense: if the light feels hot for comfort, turn it down.
For casual readers, some observations that hopefully make more sense out of the runtime curves:
Turbo: Starts at 6000 lm, steps down to 2000 lm. The light works fine after a 5 minute continuous Turbo hold during my testing, but it did get toasty and obviously 5 min hold is not recommended. In actual use, in general I use it for 30 sec to a minute duration. You could hold Turbo for longer duration but common sense here, it’s a hot LED in a smallish light. If light gets hot, turn down brightness.
Level 4: Start at 3200 lm, step down to 2000 lm. Fantastic sustained output and my favorite mode (I like higher brightness). During my walk outdoor @15°C, I could hand hold without problem. With higher ambient temp, obviously it may get hotter. It also appears a lot hotter during indoor testing/white-walling vs outdoor walk.
Level 3: 1040 lm. Runtime is a straight line across, no step down or temp problem.
Level 2: 310 lm, no problem
Level 1: 40 lm, ditto
Moonlight: 2 lm, ditto
The M21K is actually giving my number one flashlight Firefly E04 Surge with FFL505A Rosy tint a run for its money. Stiff competiont to say the least. A summary for potential buyers, first, what I like most:
1. Among the highest power LED that has warm CCT and fantastic tint. My review of this light and LED would begin thus “About the tint…” I suspect this is Simon’s primary goal, and he got it down cold. Really, would have been a disappointmen otherwise IMHO.
2. Good brightness, but more importantly, INSANE sustained brightness. At level 3 (turbo-2) @ 1000 lm the light is absolutely cruising with a straight across runtime, barely warm. But personally I LOVE mode 4 (turbo-1), 3000 lm. This is the first light in my small collection that in 15° C ambient, MAINTAIN 3000 lumen after a 10-15 minute walk. With light handle perfectly holdable with un-glove hand.
3. Hotspot size: largest I’ve seen in a TIR optic. Beautifully wide field of illumination
4. Thrower: A surprise - this light is a borderline super-thrower: around 750-800 lm. Doesn’t make my artificial 1km mark, but the large hotspot is bigger than that of the super throwers. And similarly sustains brightness longer in Turbo mode. Must use common sense here though: if it feels hot in your hand, turn it down.
5. Size - Both negative and positive: This is a medium-large light, jacket pocket strictly. OTOH it is a smallish large size 
. Kididng aside, yes it is short, and the head is smallish vs lights such as Wurkkos TS30S, etc.
The OCD collector in me is planning to buy a second M21K before possible problem with tint lottery sets in. But this new color is going to make me wait. @Simon_Mao, when please !?
The camo green is my favorite. But it’s not just about liking the color: anything to make it easier to pick out the light I want among the multiplying rows of lights.
Per Simon, the above colors should be available in March. Players… get your credit card ready.
@Sirstinky 's excellent 1lumen review of his M21K w/ LHP73B 5000k (vs my 4000k version) is HERE. A cool 4.5 star rating. Some good info:
Output 6200 lm (My 4000k version measures 6000 lm, so not much of a penalty for the lower CCT.)
Throw 720 m (M21K is an excellent thrower.)
CCT 5000k
Duv 0.0098 @ Low Level, 0.0063 @ Turbo (On the “highish” side, vs my 4000k LED that measures 0.0000, perfectly neutral @ Turbo.)
Runtime:
1100 lm level: 2 hr straight line, this being a Buck driver.
3300 lm level: sustains 2000 lm, among very highest of non active cooling light.
6200 lm Turbo: use common sense, turn off when hot.
It’s an impressive flashlight for sure, and even more so when you consider the price: $40 thereabouts with the best battery on the market currently. It doesn’t quite beat the ArmyTek Viking Pro for sustained output over time (short runtime), and it’s not as refined as some other lights I’ve tested, but for the price vs performance overall this is the #1 flashlight (as configured) on the market now. Did I mention it’s $40? As fsr as the E04? I still like that one better…just for the Anduril2 UI and beam.
Agreed about Firefly E04 (title of thread was just a cheap clickbait ). I just have to shine that light at any plant and (particularly) red flowers to marvel at the color rendition of the FFL505A LED. Colors pop, saturation off the chart.
But yeah wow 30 buck for this M21K is INSANE. I don’t know how Simon does it considering the on-going inflation, but I ain’t complaining. Great brightness, incredible sustained brightness, great tint. IMHO, recommended without reservation, for enthusiasts.
PS for potential buyer: Make sure to order it with the TENPOWER 50XG battery, the current champion.
