Disregard the post by Collects. With all due respect he doesn't understand what the question is. In order to reduce the barrel/cylinder gap on a revolver with no other problems, you must remove the barrel. Chuck it in a lathe and turn the barrel shank back a few thousands and refit everything. The barrel must be indexed and then the gap fitted properly. When I went to armorers school the gap was .003 - .010. Now S&W has increased that spec. After you get the barrel/cylinder gap right and my opinion is .004 is right to start...then you will probably have to refit the extractor rod and maybe the center pin because you will have moved the lock toward the frame by the same amount that you removed on the lathe. There are specific tools that are available to recut the forcing cone which also has to be done and also for facing the barrel. The tool to face the barrel is the best to adjust the gap to .004.

This is not a job for most local Smiths who have no formal training or machine experience. If you take it to one of them to have it done, you can plan on having your revolver ruined. However there are competent smiths who are also machinist trained that can do it. S&W can do it but may not adjust it to minimum. Also, if you send it to S&W and they have trouble getting the barrel off, you won't get your gun back.