I think it depends more on the individual gun that it does on the chambering. Like some of the other posters have said, I've seen .357 revolvers shoot extremely well from a Ransom rest with .38s, so I don't believe that the extra distance in the cylinder is that big of a deal.

That being said, I do suspect that the K-38s made around the 1950s are probably the most accurate 38/357 revolvers made. I've got several, and have tested them in a Ransom rest, and they are amazing.

That's not because of the chambering, but because I think the factory knew back then that these guns were often purchased by very picky Bullseye shooters who wanted the best. Smith was competing with Colt's Officer's Model.

Therefore, my hunch is the S&W factory took pains to make sure these K38s were very well fitted.

For example, Kuhnhausen states in his S&W Revolver manual something to the effect that most factory Smiths won't pass a match range rod--except for older K-38s, which often will do so.