My first question is: is the gun clean? Carbon and powder residue buildup on the cylinder face and the rear of the barrel can reduce the cylinder/barrel clearance. Since your cylinder slightly touches the end of the barrel, a good cleaning may open the gap between the cylinder and barrel. Normally, the gap between the cylider and barrel should be in the 0.003" to 0.006" range and can be measured with a feeler gauge.
If your cylinder will move back and forth with the cylinder closed, you do have end play and a repair, possibly with shims as susggested above, may be in order. Also you should check to insure that the crane is not bent, which can cause misallignment of the clylinder and rubbing of the face of the clylinder on the rear of the barrel.