When the 357 Magnum was introduced, S&W thought most would be ordered with the 8 3/4 inch (later 8 3/8 inch) barrel and would be used for hunting. To their surprise, they became very popular with law enforcement, especially the FBI. Since their agents are plain clothes, they wanted the shortest barrel possible. That just happened to be 3 1/2 inches and also happened to be very sexy looking. When Bill Jordan and others convinced them to make a K frame 357, they assumed they would be popular with law enforcement and some of those would be plain clothes. The 2 inch K frame 38 that they had been making for years necessitated a shorter ejector rod. A 2inch barrel wouldn't quite work with the ejector rod shroud on the Combat Magnum so the shortest practical length was 2 1/2 inch. When they made 3 inch K frames, they realized that there was little practical difference in concealability, the slightly longer barrel was more efficient with magnum ammo and the longer ejector rod was an added benefit. Since the 3 inch model 10, 13, 64 and 65 were popular, somebody thought a 3 inch model 19 or 66 would be good so they special ordered them that way. There wouldn't be enough difference between a 3 1/2 inch and a 4 inch to even tell without a tape measure since neither would be right at the end of the ejector rod shroud. The 357 Magnum was originally available with any barrel length you wanted from 3 1/2 to 8 3/8. Somewhere out there must be some 7 1/2, 5 1/2, 4 3/4 and so on. All the barrels started off as 8 3/4 or 8 3/8 and were shortened to the ordered length. Later, they standardized on 3 1/2. 4, 5, 6, 6 1/2 and 8 3/8.