The way to check a cylinder is to use soft lead slugs to see if they fit,,, not jacketed ones. Plus, you insert them from the rear & use a soft item like a pencil eraser to see if they will push through. You are approaching the issue all wrong by the picture I see.
Have you slugged the barrel as well?
What diameter are those bullets?
Remember, once metal is removed, you can't put it back,,, so I'd check many other things before I'd do any reaming.
I was going to say that photo doesn't prove anything other than that they're not really oversized. (I see you slugged them, but post this anyway just as general info for others)
If your numbers are right and it's .399", you do not want to ream it. The more metal to be removed the better when it comes to reaming. They tend to do a poor job when removing very little metal like that. That's a honing job. That little is something you could do yourself without much fuss.
If your paces are like mine that is about 20 yards. You are only expecting too much if you are hoping any random load you assemble will shoot great. If you can’t get any loads to group tighter and you are sure the problem is not operator error then I might try altering the revolver.
Before I'd alter the firearm,,, I'd work on things that are changeable & fixable. Different reloads, different bullets, & after you have a few different good groups,,, let a few other folks try them. Especially folks who know how to shoot a SA.
I have a tight shooting Vaquero in 45 Colt.
I tried many different loads, and many shot very well. HOWEVER, I tended to get "low & left" a lot. So, I got a few others to shoot it. One is a respected handgunner I call my friend. He claims I "sandbagged" him, as he shot a tiny one hole group with it. (I didn't tell him how tight it shot,,, just that it didn't shoot good for me.) BUT,,,,,, it was low & left. THEN I knew it was the gun. So, we did some minor alterations & it is more centered & level.
Just trying one load, or at one distance, or only one shooter will not give you enough info to worry about a gun problem. As a wise old gun person told me once,,, long ago; "Many gun problems are the fault of that loose nut behind the trigger."
To that end,,, I've worked hard at removing all the mechanical or ammo related issues before I shoot so I know that when I miss, I can only blame myself. Some guns do great w/o any issues, and others I've spent a lot of time working on. Eliminate the stuff you can easily change before cutting metal.