If you really want a shoulder holster, Galco's Miami Classic is made for your gun. I know it is available for the 640 .357, but if you have a .38 spl that holster may be a little big. You'll need to try for fit. Now, I don't use a shoulder holster, but if I were to get one I would not buy one that relies 100% on a tension screw to retain the gun. It should have a retention strap (a tension screw in addition to a strap is ok). My reasoning is that if you tighten that screw down enough to hold the gun in the holster no matter what you activity, you may have to pull it real hard and this would dramatically slow the draw. I have a Taurus 650 (same thing as a 640 only made by Taurus) that I carry quite a bit in a Galco Speed Paddle. Pretty could holster.
