Picked up my Ruger GP100, 357 mag. from my FFL 4 hrs ago & just now sitting down to admire her. NOW THAT'S ONE WEIGHTY & ONE GOOD LOOKING PIECE OF STEEL & RUBBER! !! I also picked up 2 books this wk by Grant Cunningham: REVOLVER & DEFENSIVE REVOLVER FUNDAMENTALS. I'll prbly digest the 2 reads, clean the powder burns from the chambers & barrel & just flat-out admire the heck out of her before I take the gun to the range. ..looking frwd to that. As slow as I am that could be awhile. lol But that's just the way it's gonna be. ..having fun, though. ..six months to study handguns before I actually picked one up. But I gotta ask: every revolver I've ever seen develops a mark around the cylinder from what I guess is called the cylinder stop..or maybe, the cylinder stop stud..not sure. But is this marking necessary? My guess is that it is. I suppose that "stop" or "stud" or whatever it's called is under spring pressure & simply is pushed into each of the 6 catches on the 100 to lock each chamber in alignment with the barrel for each shot. ..just wandering if there's a way to prevent or eliminate this mark.
A really good pistolsmith can re-time the cylinder stop, such that it won't leave a line scratched into the pistol's cylinder. But it isn't really necessary.
Merely using any gun will put scratches and other dings on it. That's part of using any tool. Just taking a gun out of its holster, and then putting it back, interferes with the metal's finish. You have to learn to accept those facts, and to keep yourself from obsessing over the issue.
The real question is whether or not the cylinder stop holds the cylinder solidly in place as the hammer drops. The second part of that question is whether the cylinder stop holds all of the chambers of the cylinder in exact alignment with the barrel's forcing cone.
The tool which answers this last question is called a "range rod," and it is caliber-specific. It is also relatively expensive. But your local gunsmith has one, and knows how to use it.
The 'drag ring' is just a fact of life. You'll see it on other Rugers, S&W, Colt, etc. The cylinder latch (spring loaded up) is drawn down out of place to 'unlock' the cylinder when it needs to turn. But it is back up into quickly to catch the next cylinder ASAP. Thus the ring is carved into the cylinder as the latch drags along ready to catch the next cylinder.
If you have a Stainless GP you can buff it off with the proper Scotch Brite pad or the proper grit sand paper (around 600). If blued, well, you're SOL. When looking at a used gun that ring can show the amount of wear or use the gun has on it along with other clues.
When the trigger is fully cocked, the cylinder will have 'some' rotational play. Again, this will be an indication of original construction and wear.
You should also check for the amount of fore/aft play of the cylinder when the gun is fully cocked. That is more important as it checks the gap between the barrel and cylinder. Excess movement here allows too much of the burning gases to escape when the gun is fired vs. pushing the bullet down the barrel. I think the proper gap for your GP is .005"-.008" but don't quote me. After you handle a number of guns you'll learn what is normal vs. what is too much.
This working play is required to make manufacturing reasonable and for heat expansions.
By the way, congratulations on the GP. I have one and with reasonable care your grandchildren will be shooting it. I've never heard of one breaking or going out of time like some other brand might (who will remain nameless).
The worst thing that too great a gap will do is "lock up" the cylinder, and keep it from rotating.
That's because a big gap, even if it seems to be only at the front of the cylinder, actually lets fired cartridges forcefully stick too far out the back end of the cylinder, rub against the gun's recoil plate, and make turning the cylinder very hard, potentially all the way to impossible. This effect is called "cartridge setback."
Jean and I inherited a revolver from Jean's dance mentor which exhibited exactly that fault.
The first shot was fine in all respects, but the second was hard to let off, and the third was impossible. It took me quite a long time to realize that if there's a big gap at the front of the cylinder, and the cylinder is free to slide fore and aft, there is a terrible result at the back end as well.
The fix, however, was dead simple: You just accept the slightly-too-large gap at the front. Adding a thin washer (a cylinder shim) to the front presses the cylinder to the rear, preventing fired cartridges from setting back. This fix worked like magic.