It would help if you were more specific with your information:
Is it a bare cell (green) or has it got a third party wrapper on it?
Is it protected or not?
Is the light in question the original H600w or the recently released H600w Mk2?
I have the original H600 (mine's a neutral-floody) and I also have some protected 18650B at home which have clear shrink wrapping so you can still see the green cell..these are fairly common AFAIK.
Anyway, I have no issues with fit. Some cells are a little tighter than others but there is absolutely no force required to insert or remove them. One or two will displace the air inside the light and make a vacuum but a little shake has it out with relative ease.
The only concern I have is overal length being a bit long due to the added protection circuit so just stick with my 3100mAh AW brand cells for the H600 and use the others in my Predator which fits everything I have tried and still has a slight rattle!
I have four of them. Not third party. They are green with a transparent plastic around.
They are protected, which makes them .2mm wider.
Light is H600w MKii, newest.
The problem is that they are too wide. If i push hard, they come in, but I dont dare to do that, since I may not be able to extract them. However, unprotected Panasonic 3100 fit perfectly.
M.