Yes, the Classic is nominally 100mm longer than the Carbine, and the FAC tube is another 120mm or so longer again.
The FAC version has a shroud around the barrel. The barrel itself is the same size as normal. 12mm diameter or slightly under, mine measures 11.92mm.
The brass extension sleeve in the photo is 15.9mm diameter and fits the FAC figure of 8 clamp perfectly.