If the cylinder stop ball and cylinder notches are in tact and undamaged, you may be able to let the stop out a bit and solve the problem. (S&W calls this, "stop doesn't hold") Shooting debris embedded in the stop notches can also cause issues, so make sure the notches are clean....use a toothbrush and a little solvent to get down into the bottom of the notch and clean out any dust and dirt there.
As mentioned in the thread above, a small amount of material can be removed from the adjustment step just behind the ball of the stop. This will allow the cylinder stop ball itself to rise slightly higher, and more fully engage the cylinder notch. Approx. .003 - .005" of material is available on the step.
