You don't move the barrel to adjust barrel/cylinder gap. The barrel is fixed and if the gap opens up, it is due to the cylinder moving back, and that is repaired at the end of the yoke. If you are putting on a new barrel, the shoulder is what determines where it will be when it tightens. The other poster described it pretty well. There is not normally an reason to remove the barrel.

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Louie the Lump
NRA Life