Hog ole buddy, the foward and aft movenent of the cylinder is called endshake. When you pull the trigger, the hand that indexes the cylinder will also push the cylinder foward. If you have excessive endshake ( more that .002" ), it needs to be eliminated before the gap is re-adjusted. You might have .005" endshake, your cylinder gap would be .005" . If you reset the barrel, when the cylinder is pushed foward by the hand, you' d have no gap, cylinder would rub on barrel.

To set clearance, they remove stock from the barrel face that butts up to the frame the equivalant of one thread. Then they face the barrel to give you the .005" gap, and re-cut the forcing cone.