Just hire back PK. He'll fix it.
In all seriousness we need proper rechargeable support. 18650 is the correct and obvious choice, lest I need to remind anybody that the original U2 had native 18650 support and second revision ones had a removable plastic sleeve to step down to 16mm diameter CR123As.
18650 is the norm everywhere else and has been for years. 18650 cells are in every laptop battery that hasn't switched over to Li-poly flat cells. 18650 cells are in every teenager's vape pen. 18650 cells are used in all of the Tesla S cars. 18650 cells are used in nearly every battery bank. It's a standard, and it's not going to go away any time soon because it's a good size relative to human hands for items handled by humans.
I would normally tolerate 16650 (which I do use) but there is one manufacturer (Sanyo) that still bothers to make them, and only one company (Keeppower) that still wraps and protects them. I don't even want to get into capacity wars but the 16650 is positively anemic at 2500mAh with a 2C max discharge rate. From March til June this year, 16650s were scarce whereas you could still find 16340 and 18650 just about anywhere online.
That is not a supply chain I am comfortable in investing my lights budget into (which I already have, for the handful of Surefires I have on "life support"). And Surefire would be beyond stupid to think that 16650 is a form factor to base future lights upon, given its relative scarcity.
The reason 21700 is now the big fish in the battery world is because of Tesla. Tesla wanted higher capacity and higher current than 18650 can handle, physically, so here we are. 21700 is a nice size as well and because it is only 3mm wider and just a hair longer, we get to a funny situation where 18650s work just fine in 21700 lights, though there would be a rattle. But, Surefire is, and always will be, a CR123A company because of military supply chains, and 16mm CR123As without a sleeve are gonna bang around inside an 18650 light to an unacceptable level. And no product manager over at Surefire is going to green light a project that uses 21700s with a sleeve for CR123As so some military grunt can lose the sleeve and make the light useless.
So, back to 18650 support. It's still going to be a painful path for Surefire. They were always designed around 16mm cells and if they move to 18mm cells as the entire industry has done, the entire legacy Surefire Lego system goes out the window. Not that they were that interested in maintaining it anyway.
How can Surefire modernize the C series? The C-series gets too thin at the threads where the o-rings are cut to really support 18650 to the durability standards Surefire has for their duty lights. People bore these things out, and if they aren't good at boring and go just slightly off-center they can get too close and bam, paper-thin weak o-ring channels.
Surefire took a crack at this by widening the body-tailcap interface on the Fury DF. You can see they did a crummy patch job, because the Fury maintains a C-size body size but takes a wider, non-standard tailcap, which looks awful.
And I believe the Fury uses the "X" series bezels... taken from the G2X and 6PX. The C-series head threads can't be kept, because not having to encapsulate a P60 means the construction is too complicated and too deep for heatsinking with an integrated LED and light engine. So the C-series is dead, all around. The only thing that can stay is the 1" body diameter, which has to, because it's the standard mount size in tactical lights.
Let's look at the E-series. Surefire hints to us what they're after with the M600DF Scout. Things actually look up here! The E-series cannot swap cells out of the tail because there is an inner collar that narrows the ID of the body. This is again, to make up for what would otherwise be too thin of an o-ring channel. It turns out to be the E-series' saving grace - the tailcap does not care one lick what size the battery is, because unlike C-series, it never tries to wrap around the battery section! So the E-tailcap can stay, although they'll probably have to (externally) get thicker to match a fatter body section.
That means the head interface has to change. The ever-fattening 1.1" E-series bezel has enough meat to take an 18650 without changing the OD, but the ID has to change. On the M600DF, so it did. I don't own one of these guys, but looking at the photos it looks like Surefire did okay.
So the M600DF looks like the path forward. I think it looks weird, and Surefire doesn't seem to think it deserves a new model designation (the bezel says KE2-DF, so what's the next series, the DF-size?)
But with all those compromises, and bear with me, thank you for sticking around this long, and I can't believe I still can write this much about a flashlight, Surefire needs to design a new system from the ground up in two sizes. C-series is dead and E-series is chock full of funky compromises so you might as well leave it behind. If you ask me, the 1.1" and 1.25" bezels should share a collar size, which should look just like the M600DF, and I guess we can keep the E-series tailcap, although new ones with thicker walls will need to be made to not have awkwardly skinny tailcaps.
I relinquish my microphone now.