Quoted:
There is a threaded section about two inches from the rear of the tube(receiver end) Unscrew this portion, the remaining two inches is the barrel nut. Use your armorers wrench to take it off. You may have to heat the forward portion of the tube to take remove it.
Does not look like a 2-piece tube in the OP's pics. Most DPMS FF Tubes are 1-piece. It is aluminum, so use heat from a high-output heat gun or a propane torch in the receiver area of the tube, then use your strap wrench to break it loose. If you don't care to save the FF tube then just use a big-ass pipe wrench. Be sure and properly support the upper receiver when loosening the 1-piece tube so as not to distort or damage your receiver. Also, be sure to remove the gas tube before attempting to loosen the handguard tube.