I have the wood stock version.
On mine, there are two screws that come in from the side of the barrel band. Under the screws are plastic balls that squish against the longer air tube and the shroud. You need to loosen/remove these before unscrewing the shroud. I have no idea if the plastic stock version is configured the same way. Not loosening these screws will make it difficult to remove the shroud. I know you mentioned the bushing that is locked onto the barrel but, maybe that piece is not the problem???
Also on mine, the rear trigger guard screw is one of the fasteners that secures the action in the stock. With this screw loose, and the shroud removed, you can watch the barrel rise as you tighten this screw (compare against the inside of the barrel band, and the distance from the air tube). The more you tighten, the higher the barrel rises. If you take the action out, you'll see that this screw tightens against nothing....except a thin section of plastic next to the hole.....which acts like a fulcrum for a lever. I put a spacer block in there, so the screw locks up solid, without causing the flex/lever/fulcrum issue. Might be worth looking at. Again, not sure if this applies to the plastic stocked version. After doing this mod, the barrel is very close to centered in the shroud without installing the bushing you mentioned (even with sag from the barrel weight/flex).
