S&W borrowed Dan Wesson's interchangeable barrel concept, albeit losing user interchangeability in the process. The S&W barrel is threaded into the frame. The muzzle end of the barrel has a flange that holds the shroud in compression between the muzzle and frame. The flange is in place of Dan Wesson's muzzle nut. The barrel is screwed in or out of the frame with a special tool that engages the rifling. An ersatz method is to center a quarter inch square rod in the bore and fill it with cerrosafe.
This is a good description. Notice the S&W design uses a fixed flange on the muzzle end of the barrel to hold the shroud in compression. The barrel is in tension, being pulled by the frame on one end, and the shroud on the other. Karl Lewis designed the Dan Wesson also to hold the barrel in tension but his design used a nut that allowed the owner to change barrels with the included barrel nut wrench.
The chief advantage of sleeved barrels, particularly without user interchangeability (to change lengths), is for the maker, in this case, S&W. To get a barrel installed correctly, the manufacturer, the gunsmith, has to thread the breech end of the barrel so that it comes to the correct tightness (torque) with the top of the barrel (sights) aligned correctly at 12 o'clock. They can then cut the barrel face and forcing cone to the correct length for the specified barrel-to-cylinder gap. If the sights end up canted at the correct torque or the barrel-to-cylinder gap is too big, they have the unthread the barrel and cut an additional thread so the barrel can be screwed in a little more to either get the alignment or the gap correct. It's time consuming to do it correctly and any mistake means doing it all over again. With the sleeved barrels, they screw the barrel in to the correct torque. There is no concern about aligning the sights because those are always aligned perfectly on the shroud. An assembler can be made competent to do this with minimal training. It's easier to do and the result is consistently good.
There is some conjecture about the tensioned barrels being more accurate. Tensioning barrels itself does not make them more accurate. If that worked, we'd see it in rifles. The Walther WA2000 was a costly experiment in that and it did not meet with practical success. If there was an advantage to be gained, nobody in precision rifle disciplines has seen it.
Revolver barrel accuracy can be affected by the barrel getting deformed during the rifling process and during the installation into the frame. Different processes to form the rifling in the barrel can result in different tapers and deformations in the barrel, and the barrel's profile can also affect the end result. This is a whole other topic that is relevant not just to revolvers, but all kinds of handguns and rifles. A person wanting to know more can read plenty about hammer forged, broach cut, button, ECM and other rifling processes. The process S&W currently uses on its revolver barrels is ECM. With this method, they can get a good result regardless of the barrel profile. There are other methods where having a round barrel would be a big advantage. Cold hammer forging, for example, is really best done on round barrels. I can't imagine trying to hammer forge rifling in a funky-shaped revolver barrel with the full or half underlug and with the boss for the front sight and the flat top. It could easily be done to a round barrel sleeve and then that would slide inside a shroud that has all the funky shapes. So with hammer forged rifling, what is typically done is the rifling is forged on a fat round (cylinder) barrel blank, and then the shape, whether it's just a taper or anything else is whittle away after rifling. That process can change the internal taper of the bore and it has to be carefully planned and executed. If you look at the muzzle of an Anschutz rifle, the barrel is usually bulged. They forgo tapering the end of the barrel because they know doing so will open the bore up which is the opposite of the desired "squeeze bore." Well, I wrote that S&W uses the ECM process and unlike forged rifling, ECM doesn't induce the kind of stress that makes machining the barrel after rifling a risky process. What's more, ECM can be used to rifle the barrel after it's been fully shaped. However, screwing the barrel into the frame can also deform it. It can create tight spots or bulges. This is true not just for S&W but for any revolver and most revolver makers have had trouble in this area at times. I suspect the old process S&W used to pin barrels was better at avoiding constrictions and bulges, but it barrel fitting, cylinder gap setting and barrel alignment complex enough that it took a very skilled machinist and was hopelessly impractical for an assembly worker. There are valid solutions to the problem other than pinning the barrels, but some people will always prefer when they did everything the hard way.
Another advantage of sleeve and shroud barrels is that different materials can be used for each. The barrel can be a stainless steel, but the shroud could be aluminum or some other lightweight material or a material with other desirable qualities. So not only can it be complex in shape without potentially screwing up the bore to get that shape, it can also be a totally different material.
