Pretty much all of the tanks on guns would expand enough to potentially cause issues when attached to a barrel. If you get a chance try wrapping masking tape tightly around an empty cylinder before filling it- gives a pretty good visual indication of the degree of expansion. I suspect the steel cylinders might actually 'bulge' a bit more than the carbon wrapped ones.
Guns like the AA S400/500 get away with bands by attaching them to the very end of the cylinder at the fill port, where they can avoid the circumferential expansion exerting upward and downward force.
The Weihrauch HW100 has a thick steel barrel and a
very thick steel cylinder and even they attach the band to the stock, with no support provided by the cylinder itself. Even so they put an oring in the hole for the barrel as well as cylinder to help alleviate any strain that may occur.
Unfortunately with FX bottle guns it is much harder to put a band onto the end, for obvious reasons, and again the STX system's structural weakness vs other barrels man that relatively minor forces would have a bigger effect vs the above mentioned systems/guns.
Perhaps one could make a low profile support 'bracket' of sorts attached to the front of the scope rail or to a stock attached picatinny rail (again, wood vs metal expansion with temp can cause issues) and extending forward to support the barrel, but that would look pretty frankstein

Goodness only knows if that would work either.