...or even carbon fiber. I did a QB based pistol that used a chunk of graphite golf club shaft tensioning a piece of cr 2200 soda straw barrel. It weighed very little. A rifle version I did for a 2260 weighed about half what even that skinny 2260 crosman barrel weighs...and it was at least 4x as stiff.
The heaviest single component in a QB 78 is the barrel, but the rear end cap can be trimmed and dished, striker lightened,the spring support flat sided, and the stock hollowed out or skeletonised, and slimmed.
I suppose you could even shorten the gas tube and run a single cartridge, but I think those 180 styled short tubes are just awkward looking.