Originally Posted by armorer951

The "rear gauge" as S&W calls it, is the area between the breechface and the rear of the cylinder. It's measured with the cylinder closed, in the area between the cylinder and the firing pin bushing.

End shake on the yoke is a symptom of an improperly fit or worn interface between the button on the end of the yoke stem, and the yoke screw in the sideplate. If not fit correctly, this causes the yoke/cylinder assembly to move forward.....and thereby, opening the rear gauge.

End shake on the cylinder is gauge between the end of the yoke barrel and it's bearing surface inside the cylinder. As these surfaces wear, the cylinder can move forward on the yoke barrel and open the rear gauge to the point where the revolver begins to misfire, due to failure of the end of the hammer nose to adequately reach the primer.