brought the upper from the M16HBAR home today and took some picks with it on my RRA lower. the bipod is from a M60 machine gun. the muzzle brake is something colt developed but only appeared on the rare hbar uppers as you can see its a full heavy barrel all the way to the chamber. its not the typical hbar profile. the most material is where the hottest gasses are in the barrel itself. supposedly this greatly increases the time of sustained fire and full automatic fire. i think im gonna keep that barrel even if the sherriff doesnt sign off and i have to sell the gun. ill replace it with my spare colt m16a1 barrel. there is no special marking on the lower saying its a hbar model. colt just slapped that upper on existing m16s and m16a1.