I've read that when the revolvers were assembled final end gap was opened up with a file if it initially was too tight. Is that all there is to the story?
Howdy
Not quite. Filing is a slow process, much too slow to be done in production, and it requires a lot of skill to get the gap right without removing metal unevenly.
Somewhere I have a photo of the apparatus that S&W used to use to trim the butt end of a barrel to the correct length, but I can't put my hands on it right now. When the barrel was screwed into the frame the portion of the barrel that protrudes into the frame was purposely left a little bit long. The barreled frame and the cylinder to be used in the gun were placed on a special jig. The frame was secured down, and the cylinder was placed a few inches away on the fixture. A broach was used to trim the barrel to the precise length needed. A broach is a bar of steel with large, sharp cutting teeth cut horizontally across the bar. A broach like this cannot be used by hand, it is mounted in a power press that pulls it up or down past the workpiece to remove a precise amount of metal without any misalignment.
The way this fixture worked, the frame and cylinder were secured to the fixture. A 'feeler' or probe was then touched to the cylinder. This adjusted the amount of cut the broach would make to be perfect for that cylinder. Then in one stroke the broach removed the precise amount of steel from the butt end of the barrel. When this cylinder was later assembled to the frame, the gap would be the precise correct amount needed. But the gap had been set for that particular cylinder.
I dunno how it is done today. I suspect a CNC controlled cutter accomplishes the same thing. But that is how it used to be done, the photo I saw was probably from the 1940s or so.
*****
You need to understand that cutting the forcing cone and tuning the barrel/cylinder gap are two completely different things.
The forcing cone is the conical shaped surface on the inside of the butt end of the barrel. Look into the butt end of your barrel. The unrifled portion is the forcing cone. It is slightly larger in diameter at the rear than it is where it merges with the rifling. It functions as a funnel. Cylinder alignment with the bore in a production gun is almost never completely perfect. The forcing cone functions as a funnel, gently guiding the bullet from the cylinder into the rifling without marring it or damaging it. Without a forcing cone, most production revolvers would probably spit lead because the chambers are not perfectly lined up with the bore. Revolvers have had forcing cones from the earliest days in the early 1800s.
A forcing cone cutter is designed to cut the cone to a precise angle. Some guns leave the factory with forcing cones of various angles. 11 degrees is usually thought to be ideal. A proper forcing cone will keep leading to a minimum. Also, with an older gun, the forcing cone can become pitted and have a rough surface. So a forcing cone cutter is used to remove a tiny bit of metal to clean up the cone. If the cone is cut to excess, the wall of the cone will become very thin. That is when another cutter is used to remove a tiny bit more metal from the butt end so the wall will be a tad thicker. Doing this of course increases the barrel/cylinder gap.
Both of these cutters are used from the muzzle end of the barrel. A guide rod is inserted into the muzzle. The guide rod has a handle on the end to turn the tool. The cutting tool is screwed onto the other end of the guide rod. This arrangement keeps the cutter aligned with the bore so the cut will be perpendicular.
The only difference is, the forcing cone cutter is inserted into the forcing cone, the chamfering tool stays outside the barrel and cuts the butt end. The tool is turned slowly by hand with pressure applied forward.
Here is the cutting tool used to trim the end of the barrel.
http://www.brownells.com/gunsmith-tools-supplies/handgun-tools/barrel-tools/chamfering-tools/90-chamfer-cutter-prod26154.aspx
This is a forcing cone cutter.
http://www.brownells.com/gunsmith-tools-supplies/handgun-tools/barrel-tools/chamfering-tools/11-chamfer-cutter-prod26151.aspx
I strongly suggest you have a gunsmith who is well versed in S&W revolvers take a look at yours, to tell you if anything needs to be done to it at all. I have
dozens of S&W revolvers. I have never had to trim the barrel on any of them.
P.S. A friend who is a crackerjack gunsmith has cleaned up the forcing cones on a couple of my very old revolvers. They were a bit pitted, so he cleaned them up a little bit. He did not remove enough metal to appreciably narrow the wall of the forcing cone, so no material had to be removed at the end.
