Using Google Search at the top of the CPF page for:
aw 2600 eagletac
provides:
http://www.candlepowerforums.com/vb/showthread.php?t=238030
"note that the new AW 2600 flat top doesn't work on EagleTac T20C2 unless you attach a small magnet either at the flashlight head contact or the battery positive end to make the contact."
(emphasis added)
due to the protruding/raised lip reverse polarity protection on most of these lights - flat tops will not work -
conversely I have not found a button top cell that does not work regardless of capacity (ie.length)...
The M2 series on the other hand should work either way as the battery carrier does not contain the reverse polarity protection ring/lip that the other models have.
I'd be leary of using a magnet due to potential for cell shorting against the metal reverse polarity ring =