How do ya think they get ANY barrels?  

All M16-family barrels have to start out full diameter so the aft end will thread into the receiver.  To get them smaller diameter they all have to be turned down.

HBAR barrel is just a cheap way to produce barrels.  Much less machine time.  Time honored advertising technique is to advertise a liability (needlessly heavy) as a benefit!   Golly, those barrels on the rifles at the National Matches at Camp Perry are all very heavy, HBAR must be a good thing!

-- Chuck