I just got the new SOF and read the 4 page article about the XM8. Sounds like a good deal to me, IMHO.
One of the unique features: "...its ability to be quickly reconfigured from one variant to another to meet changing mission-specific requirements, and this includes a potential capability for caliber conversion to such rounds as the 7.62x39mm, 5.45x39mm, and the new 6.8x43mm cartridge."
With a cartridge choice other than 5.56 then the barrel configs make a little more sense considering the ballistics with a 5.56 round and shorter barrels.
At the end of the article there is a little teaser in there about a hush-hush modified G-36 upper receiver group to fit an AR (M4) lower receiver. Piston driven.
Sounds like a good start to me....
Reference: Johnston, Gary Paul. (2003) "H&K's XM8 Modular Assault Weapon System", Soldier of Fortune magazine, February 2004, Vol. 29 No.2, Pages 62-65.