Smith & Wesson’s cylinder stop window (the little slot in the frame under the cylinder through which the cylinder stop protrudes) is located in the center of the frame. In larger revolvers (K, L and N frame) the locking notch in the cylinder is directly under the chamber. This is O.K. so long as there is enough metal in the cylinder wall so the notch doesn’t cut into the chamber. On smaller, 5-shot cylinders this cut comes between the chambers, and this allows a smaller diameter cylinder. Colt’s cylinder stop (they call it a “bolt”) is located off-center so they can use a cylinder that is only slightly larger then a “J” frame Smith but still hold 6 cartridges. The off-center bolt locks the cylinder in the web between the chambers. All of this started back in the 19th century and is still carried down to us today.
So the bottom line is, if the 6th. shot is really important to you and you want the smallest possible snub you’ll need to look for an older Colt. If you can be satisfied with 5 shots (6 in .32 caliber) a S&W, Taurus, Rossi or Charter Arms will probably fill the bill. If size isn’t all that important the snubs built on larger frames (S&W “K and “L,” Ruger, Taurus, and other may meet you’re needs. As a rule though, I prefer a 3” or longer barrel on those bigger guns.