Bush HBARs have been advertised at 8.5lbs - now I see they are advertising them as 8.3lbs - but they advertise the flattop version at 8.4lbs.
When a Flattop is 0.2 lbs lighter (sans handle) or about 0.4 lbs heavier with the handle (you can see this also if you compare the weights of the 16" carbines (flattop vs A2 handle).
However note this, a real-deal M16A2 weighs 7.5lbs (with A2 profile barrel). The 20" HBAR barrel weighs 1.0 lbs more than an A2 barrel. That should put the 20" HBAR in the 8.5lbs range (the number they used to quote) - the only differences in weight being the bayonet lug, auto sear, extra disconnector, and flash supressor (which is mostly offset by the heavier barrel where the A2 is normally threaded).
So something is hosed with the current weights they are publishing.
Basically fluting will reduce a 20" BAR by half a pound. So a 20" HBAR will weigh in the neighborhood of 8.0 lbs. If you want lighter go with their A2 profile barrel - you won't notice a difference in accuracy, just weight.