I have found a couple of swamped barrels that seemed to not shoot center. I'm not sure if they were damaged in shipping or if they got some kind of stress bend from the swamping process.
As for the thumb and emory crowning, I feel, it is to slow, and to hit and miss. I much prefer a fine abrasive ball shaped stone in a variable speed power source. Oh, and a good eye, and a steady hand, helps.
Coned barrels are like talking about religion, and politics, at the same time. I can't tell you how many shooter on both sides of this argument I've seen literally foaming at the mouth arguing their side of this subject. I've never been convinced it does any good. Done right it doesn't seem to do any harm either, so if it feels good, do it.
i had a Mowrey squirrel rifle a few years ago, that shot about a foot to the left at fifty yards, and only shot center when the sights were hanging of the edge of the dovetails. An old shooter at a rendezvous that saw my predicament said he could fix it, so I handed it to him. He grabbed a file and started working on the muzzle to my horror. He calmed me down and told me to shoot it. I did, and it was much closer to center, after resetting the sights. He filed some more, and we shot it again, and it was now shooting center. The muzzle looked like it had been cut off in the dark by a blind man, but it shot fine.
Hungry Horse