Interchangable in only one direction. 357 doesn`t go in 38.
Not true. A .357 cartridge will slide right into a pistol marked .38 or .38 special and it will fire. The problem isn’t that the .357 cartridge won’t “go in” a .38 the problem is that a gun made for .38s or .38 specials is not made to handle the pressure. It might blow apart in your hand.
Please note interchangeable cylinders of .357 Mag (9mm) and
.44 Colt(.45 Auto):
http://www.ruger.com/products/newModelBlackhawkConvertible/models.html
Ruger also makes a Convertible .22 LR/ .22 Mag.
http://www.ruger.com/products/newModelSingleSixConvertible/models.html