Don’t do it. 4 x 18650 has to be sooo much better. The least you’d have to do is make a contact plate in Eagle… And the tail end is extremely easily modified.
I realize the capacity benefits, and making a custom driver isn't an issue, but 4x18650 without gluing the head shut or adding a set screw has too much possibility for very "heated" mistakes (I think you know this from personal experience).
I think the M6 would be a great host for it.
18350’s would be an easier build but would be a bit of a pain to manage all the batteries which would need to be done often. 18650’s in serial would be much better.
Edit: Perhaps a light with a removable battery holder would be a better choice.
Hi Beam, the problem is that we are working with an existing light. The distance between the driver and springs is a set figure. We can’t just start building battery carriers and shoving em in, they won’t fit. So there is no universal easy solution. Have a new tail cap made and this would be very much the way to go. Short of doing that, it’s a problem.
TK75 would be pretty sweet, but that's too expensive.
Oh, my bad, I thought you could use some old beer cans to make a battery carrier, and if you were to hit it real hard with a sledge hammer you might get it to fit.
I guess being employed by IBM for 14 years as an electromechanical engineer didn’t teach me a thing did it.
It’s always a good idea to leave sledge hammers alone when beer is involved. 
Of course, I could not know where you’re coming from. Please feel free to draw us up a battery carrier for the M6. Oh yeah, it’s not a simple tube.
Please dont cripple it with 18350’s. Do this instead: Mod: My SupFire M6 "BMF" edition (new beamshots in OP). M6 Goodness!
You mean this?
I don't trust myself to not screw up at some point with it, let alone other people who may pick up the light. I think that a dedicated battery carrier may be in order. Bore out the M6 tube and slide it in.
Just to be clear: I'm not mocking Mike C's build, in fact I think it's quite cool, but it's just not for me.
The nice thing about the M6 is that it can be thrown like a hand grenade if it starts smoking. 
Boring it out for a cell carrier sounds like an interesting idea, and certainly far safer to keep the dead-short gremlins at bay.
If you plan on making a dedicated battery carrier for it, why bore out the tube? Why not make a 4 leaf clover shape to match the contours of the housing, with 1 or more smooth connecting rods running down the center of the batteries. Once inserted the batteries would lock the carrier in place to prevent twisting of the top plate. There appears to be enough room to insert 2 possibly 3 smaller connecting rods in the center of the batteries if twisting is of concern, however if engineered properly the top plate would lock into the grooves cut into the housing for the batteries and twisting of the battery carrier would be an impossibility.
I agree Hi-Beam, that is pretty much what I was thinking about too. With the clover leaf plate to convert to 2S2P the button top cells of 2 would be in contact with the driver as usual, the other 2 would be in contact with the springs. That should work fine and removing the battery tube for changes wouldn’t impact anything, it would be necessary to remove the tail cap to get to the other 2 cells though, right? Unless the carrier would slide out, which should also work. Maybe a hexagonal shaped aluminum rod in the center of sufficient size to allow the use of 2 or even 3 small screws to prevent twisting when out of the tube?
LCK-LED just emailed me also about the 50s and said they will be in stock on the 26th of this month.
Aren’t they in Germany?
Mouser will have 50’s in stock the 23rd of next month, they say.
Are these the warm ones at 3000K?
There’s only 0.30” between the cells in the M6. Maybe a square shaft?
Ok, looking at the light, why not simplify? A block of pcb material crossing the driver end could have contacts to touch the negative end of the blocked cell and also the retaining ring. The pcb with springs in the tail cap could then be modified to tie the 2 groups of cells together for series. Batt + would be the two button tops, as usual, Batt - would contact at the retaining ring. That one simple block would do it all. The 2 cells that are inverted will sit deeper in towards the tail cap as the springs won’t be on that end, but on the block end instead. Smaller springs perhaps, to facilitate loading the cells to start the threads. The cells themselves are held in place within the tube by it’s machining.
I have a 12” x 12” x 1/2” thick pcb plate of flame resistant material made for electronics. I think I’ll give it a go with that or with some 3/4” thick G10 I have.
The carrier would be removed to insert all four batteries, no need to remove the tailcap. The batteries would never come into contact with the driver or tailcap, contacts would be built into the top and bottom plates of the carrier. The existing bottom spring contact plate would be removed to create more clearance.
A solid square shaft would be best as long as it is oriented correctly during assembly.
That is very tempting…. but I think i’ll wait a few weeks to see if the cost comes down at all.
DBCStm, I think that the TK75 extender kit is a go. Just need to get a new cap or get the tube rethreaded.