Some pics:
Some low profile gas blocks:
Left - Mongo special
Center - KAC
Right - LaRue
Three FF tubes:
Left - DD
Center - LaRue
Right - Troy/Samson
Finally, a pic of the fit between the LaRue tube and LaRue gas block.....................tight, isn't it?
For those who wonder...............................why is this an issue? When you are installing a FF tube that extends past your low profile gas block, you have to be able to install the gas tube and gas block first, then slide the tube over the installed gas block for final assembly. If your gas block is too tall/wide/whatever, you won't be able to install the tube, especially in the case of LaRue tubes.
Oh, and BTW, the LaRue tube is "oval" in the purest sense of the word, just not as "oval" as the DD. The LaRue is taller than it is wide..............not by much and not as much as the DD, but it is.