I recently purchased an M640DFT and a DS00 dual clicky/tape switch tailcap for it, and they don't seem to be compatible.

Has anyone else run into this or found a (good) solution?

The M640DFT works with the regular clicky cap it came with.
The DS00 tailcap works on my old M600 scout.
The DS00 tailcap works with the M640DFT if I put 2x CR123's in it.
When I put the 18650 battery in the M640DFT, the DS00 tailcap does not work.
It looks like it put some scoring or scuff marks on the back of the 18650 battery after I tried to put it in with the DS00 tailcap.

Yes, I can use the light with 123's, but it's brighter with the 18650 and I want to use that (I like the whole rechargable battery thing).

I'm not 100% sure, but to me, it looks like the DS00 battery stop collar (what I'm calling the black ring around the spring) might be too long to allow the DS00 tailcap to screw all the way down when using an 18650 battery.  Alternatively, with the tailcap all the way on, the M640DFT head can't screw in far enough to work right.

Any thoughts or solutions?  
I've sent off an email to Surefire, but it's Saturday night and I'm impatient and hoping to hear of a solution to get it all working without waiting on Surefire.

I have a M340DFT with DS00, the black ring in the tailcap looks right, I would also check the head as well.  It should have like two different sprint loaded parts, the center contact pad and a sleeve around that that has a secondary spring loading on it.  If one of those is jammed up and not moving the longer 18650 from stop the head from fully seating.

Thanks.
I took a look at the head.

The black piece doesn't appear to move.
The center copper pad and silver disk appear to move together.

I messed around with the head a bit.
It's weird.  The light and DS00 switch appear to work if I thread the head on partway, but if I do the last turn to tighten it down and seat it, it stops working.

I run that setup, but on an M340DFT, no issues at all.  I've had the switch for a couple years and it's seen a fair amount of use on varies bodies, but the 340 is fairly new.

Interesting, the contact pad arrangement on the 6v turbo head is a bit different from the shorter single cell one, it has a much larger center contact pad.  As for the black rings, my understanding is those are not supposed to really move much and are there to act as stabilizers for the battery when it/they move under recoil so the contacts don't break and cause the light to flicker.

I would wait for SF to weigh in, maybe an issue in the head or tailcap that connection is lost when tightening down for some reason.

This is a dft head I bought from a member here and the contact looks a lot different. The middle silver part is spring loaded as well as the secondary silver ring below it. They move independently of each other. The black ring definitely doesn’t move.

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ETA: This head came on an EDC1-DFT Turbo 18350 handheld, but I’ve used it on a standard 18650 df rifle light body (Surefire) with a single lead Surefire tail cap and tape switch. I’ve also used it a bunch on a Modlite 18650 handheld body with a Modlite clicky tailcap. It has a low power mode as well as the full blown lightsaber turbo mode.

Mine is a KF1-A-DFT, which is also the single cell head, but one mode only as it came as a weapon light (must be the "A" vs. "B" designation).

It has the same contact arrangement as your pics.  Not to say the OP's is fake as I haven't seen a KF2-DFT head myself, but at least on the KF1 ones, the dual spring pads in the head seem to be for the difference in length of batteries between the 18350 and CR123a.

That makes sense. Also, depending on whether you’re using rechargeable CR123s or not, there’s a length difference. The Surefire rechargables are protected and have a “cap” on them where you can plug in a usb. There’s also a small diameter difference.
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Not to be insulting, but try cleaning all threads with alcohol. I’ve fixed many issues starting with this and fresh batteries.

The rechargeable is an 18350, which is a different battery type. Similar but not the same, for those who might not know.

The light came direct from Lipseys.  I've never seen the inside of an M640DFT head before, but the packaging and everything else look right as far as I can tell, so I think it's a legit Surefire, but you never know these days.

I took the o-rings off, got the alcohol out, cleaned up the threads.  Assembled it dry, no change in behavior.

Then, I had the idea to try a different 18650 battery.  I've got Panasonic 18650 cells here for another light.  They're a hair shorter than the Surefire cell.  The light functions correctly with them.

But, I'm annoyed, I like the idea of the USB charging on the Surefire cell if I'm not near my battery charger, so I grabbed a second Surefire cell that I had here.

It appears to work correctly with the second cell.

I don't know for sure, but I suspect the Surefire cell that came with the light is malfunctioning when it gets squeezed front to back a little too much.  Maybe the protection circuit is busted, or something else is wrong?

You guys ever see this before?

I just reassembled it with the original cell and it appears to be working.
I'm stumped.  Maybe the threads were dirty and it needed a cleaning and a few assembly cycles to realize it?

I don't know at this point.

I've been thinking it over more and my tentative conclusion is that something's wrong with the Surefire 18650 cell that came with the light.

The light initially worked with 123's and with the included 18650 and clicky cap.  It worked with the DS cap and 123's.
It worked with the DS cap and 18650 when the head was partway on, and quit with the final turn or so to install the head.
The other 18650's (Surefire and Panasonic) worked fine with both caps.

I put the same 18650 cell in the light with the DS cap and clicky cap.  You can see in the photos that the cell sits lower in the body with the clicky cap.  

My theory is that, due to the different battery position/spring force, the cell sees more end to end force with the DS cap than the clicky cap and the 18650 protection circuit was somehow malfunctioning due to the higher end to end force on the cell.  (Which would also explain working when partially threaded, then abruptly turning off when the head was fully installed)

I'm not sure why it would have started working after all my early attempts.  Maybe exercising the cell got it loosened up?

I emailed Surefire about this and explained what tests I did.
Unfortunately, their guy is either ESL or didn't read my email.

To paraphrase their response: no, you need to thread everything on all the way.  The scout lights work with DS00 caps.

I'm disappointed with Surefire.  Chinese 18650 cells, poor tech support, and whatever is or was happening with my light.   I'll try giving them a call and see if they'll exchange the suspect cell.  Hopefully I won't need to speak Hindi to get the point across.

I've got an update for anyone interested.
I spoke with Surefire.  They insisted I didn't have the tailcap on and was stopping when I felt the o-ring tightening.
I was convinced they had no idea what was going on and was sure it was the battery, due to it shutting off as the head tightened on, trying it without the o-rings, and me cranking the shit out of the tailcap.

but I went back and did some more testing

If I put the tailcap on all the way, then and back it off a tiny fraction of a turn, the light will function as soon as the head is on the body making battery contact.  As I tighten the head down, it will cut off around the last turn and stay off.  This appears to be repeatable.

If I tighten the tailcap the last tiny fraction of a turn (5 degrees, maybe), the light works normally.

I was convinced I had the tailcap on all the way.  Maybe it needed a few cycles of getting tightened down to fully seat, or maybe I'm weaker than I thought.
Either way, I'm now thinking that maybe the tailcap wasn't fully seated and that it has a super short band where it works,  and locks out relatively quickly.  I've got an older DS switch and M600 scout here, I'll try and do some experiments with those and see how the behavior compares.

I've never had this issue but I will say that the tailcap will feel like it's on the entire way but usually needs about 5 degrees more rotation.   They are sealed very well.