It is my understanding that yes the barrel is extra heavy with the heavy duty hand guard. It also has a special front sight base/sight. The hole in the FSB is higher so the gas tube is straight and is also twice as thick as a standard gas tube so it can handle more sustained fire.
Scott
From Freedom Ordnance's description their is no gas tube.
http://freedomordnance.com/product/fm-9-9mm-belt-fed-ar-15m-16-upper-receiver/
"Disintegrating link blow back belt fed only system."
My bad. I thought the discussion was still on the FM9.