The tube length only changes if the gas port distance does - the STAG 10 series utilize most common DPMS LR308 based barrels, which more often than not simply use the same rifle length gas tube as an AR15.
The longer gas tubes come into play on Armalite AR10s, along with aftermarket barrels that have been specifically designed with a longer gas system in mind. If you run across a barrel that states it uses a longer than standard rifle length gas tube, they likely include a gas tube with it.
Your Faxon barrel will use a standard AR15 rifle length tube.