I have a model 620 and believe that it's the most accurate 357 Magnum that S&W has ever made. However, in order to use the level of accuracy that the 620 offers, most of us need to use some type of optical sight because the sight radius on any 4 inch revolver is somewhat short.
The accuracy of the 620 is a direct result of it's most controversial feature. That is that it uses a 2 piece barrel assembly consisting of a barrel tube under tension suspended within an exterior barrel shroud. This has triggered nearly as much animosity among some as the Internal Lock, they usually state it's been done to cut costs. Personally, having spent nearly 30 years in manufacturing as a Design Engineer, I rather doubt that S&W has saved a nickle on this method of producing a handgun barrel. In addition, in an Engineering Dynamics model, it's a superior method of attaching a barrel to the frame of a revolver in terms of stiffness.
However, there have been a few failures of the cap that is formed at the end the barrel tube that applies pressure to the surrounding barrel shroud. In addition, unlike the Dan Wesson revolver, which uses a nut at the end of the barrel shroud threaded onto the barrel tube, this cap and tube type of mounting the barrel is NOT user serviceable, or serviceable by a gunsmith. S&W uses a special EDM machined insert that engages the rifling in the barrel in order to tighten the barrel in the frame and will NOT release this tool to gunsmiths or users. Because of this, in the rare instance of the barrel shooting loose due to an assembly error at S&W, the gun will have to be returned to S&W for servicing. In addition, forming a cap on the end of the barrel tube does create a stress riser in that area which is why this type of barrel has always failed at the cap. However, a standard 1 piece barrel has a similar stress riser at the junction between the threaded extension that fits into the frame that results in what I expect is a very similar rate of failure. Basically, on the 620 when there is a metalurgical flaw in this area, the cap will snap off, on the 686 with a similar flaw, the entire barrel snaps off at the frame. IMO, in terms of the potential rate of failure, it's probably a tossup, the only real difference is in the mode of failure.
As a US resident, a trip back to the factory for service really isn't a concern for me. However, as you are in Germany, you may want to consider the 4 inch 686 as an alternative simply because you can have a barrel on the 686 replaced locally as opposed to dealing with an international shipment of a handgun for repair.
The only downside to the 4 inch 686 is the slight loss in accuracy compared to the 620, the somewhat ugly full lug barrel profile, and the slightly more muzzle heavy balance of the 4 inch 686. In your case the tradeoff may be worth the tradeoffs. BTW, IMO it's a real shame that S&W didn't put the 686 Mountain Gun into the standard catalog, it's a half lug profile and a real beauty to my eyes. It's also rare enough I've only seen pics of it.
The real question is what you intend to use your 357 Magnum for. If your going to use it for precision target matches against the Korth users, IMO the only way to go is with the 620. With a good Reflex or Red Sot scope mounted to it, the 620 can hold it's own in a precision match setting against almost any revolver that doesn't offer an adjustable B/C gap like the Dan Wesson. If your using it in Action Matches, or simply for defense, you'll probably want to get the 4 inch 686 just because you can have a barrel issue fixed locally. While loose, or broken barrels are actually quite rare, in your case it's something to think about.
As for the accuracy of the 620, the following link will provide one answer, it's a bit stunning. I've also found that my 620 seems to be VERY insensitive to 38 spl. ammo, which is mainly what I shoot in mine due to the cost of 357 Magnum ammunition. Basically, it will group under an inch at 25 yards with any 38 spl. I feed it on the days when I am shooting well.
Smith & Wesson Models 619 and 620 .357 Magnum Revolvers
