Several of the newer S&W's use a two-piece barrel & shroud system. There are two different types of these. There are those like the TRR8 that use an inner barrel tube that threads into the frame, the shroud slips over the barrel and a barrel nut screws onto the muzzle end of the barrel tube, placing the barrel in tension. This is, essentially, the old Dan Wesson method and, supposedly, the secret to their reputation for great accuracy.
The second is system is pretty much the same accept there is no separate barrel nut. The muzzle end of the barrel tube is flanged and bears against the end of the shroud. The barrel tube is threaded into the frame and tightened with a tool that engages the tube's rifling.
In both instances the barrel is being held in tension. The new Model 66 and 69 use the latter method as do some of the Scandium framed PD revolvers.
Hope this helps.
Adios,
Pizza Bob