Im a bit of an airsofter when im not tanking (no sniggering at the back that boy) so maybe able to help with this.
When i first opened up my old Tiger I assumed the drilled hole in the barrel was to reduce the velocity of the BB to a safer "toy" level. But it maybe that the air chamber which the piston moves inside doesnt have enough volumn to completely fill the barrel on the longer guns. If this happens then the BB will begin to slow down before it exits the end of the barrel as it doesnt have enough "puff" to fire it out quickly.
Also with the weaker springs used in the HL tanks compared to "proper" airsoft guns, its possible they could also suffer from the dreaded "SUCK BACK"

This happens where the bb is still travelling down the barrel when the piston starts its return stroke in the chamber, and effectively begins sucking air from the barrel to fill the air chamber.
You could upgrade the whole firing mechanism of HL using airsoft parts easily available on ebay but you'll encounter the usual problems airsofters suffer from such as: spring power, air leaks, well made polished inner barrels, decent BB's and hop up (thats the back spin that keeps the BB flying horizotal for longer). Also I might want to start wearing eye protection as a bb firing @ 300+ feet per second can give you a real nasty bruise or break the skin :-X