Fixed! Seems to be operating properly now. 
Quick clean of inner tube and inside of outer tube, inserted inner tube to a point with o-ring seated and very high resistance (55 M ohms) between the tubes, installed the tail cap, and inner + tube to outer reads open on the 60M range.
Even so, I’ll add kapton when I get it.
thanks thats a bummer… should have made the light 2 mm more to give room for even longest batteries imo 
At this point I am very satisfied with the fw21. The only suggestion I have is a groove at the inner edge of the head end of the outer ground tube for an o-ring to help maintain no-contact space between the outer body ground tube and inner tailswitch + tube. Anodizing may be good for insulation but it’s not perfect.
What size trits fit in the slots?
1.5x6mm trits
Just reviewing the FW21 for my channel now, complete fail of a light (Lumintop recommend 10A> unprotected button tops, none fit lol) and Lumintop won’t answer my messages which furthers the misery of this project.
where did you get it?
the first batch had a top long tube
Banggood.
my inner tube is 80.20mm
my outer tube 75.35mm
long batch
Samsung 50E fits well in mine.
Using the supplied 18650 adapter and running a Samsung 30q flat top until I get a 21700 battery. Battery fit and function are fine.
forgot to mention I have the M4DM4X later, shorter version
Lumintop have told me only a 75mm+ protected cell will fit in batch one.
That is why I waited for batch two.
If it was my light, I would simply stretch the tail spring to make it longer. Or maybe swap in a longer spring. Or add a solder blob on to the button top. Easy fix.
I can do that but, I won’t recommend it in a review
how long are your tubes?
85mm
WTF??
Please send me a picture of the measurement
I need to inform some people about that
it says on banggood the current batch does not fit flat tops unprotected like fw3a… so the question is , why did lumintop sent that batch to banggood? kinda useless if normal 21700 unprotected suddenly are too short… this is something u shouldnt have to worry about.
About longer tubes…
I have also first batch from BG and I wrote to Lumintop if I can get shorter second batch tube and they told me that tube is the same on both batches but second one has longer springs.
So they recommend me to swap spring for longer one.
Ive swapped the tail one for spring from led4power and all cells now fit perfectly without rattling and poor contact 
Hello, do someone know, where can I buy FW21 (not FW21 Pro)? Just can’t find it anywhere, only pro version