Usually, with the feeler gauge you are trying to find one that slips all the way through from left to right between the back of the barre and the cylinder. If you have one that slips half way through from one side, but gets stuck on the way through and won't fit in at all from the other side, then, unless the frame is visibly bent or the cylinder visibly crooked in it looking down from above, I'd be suspicious that the back of the barrel isn't cut squarely. I don't know what kind of operation they use to cut that. I'd do it on a lathe, but they may saw it.
I would check the cylinder timing, too. Drag your thumb lightly against it while it turns to be sure the cylinder stop bolt clicks into the notch on the cylinder for every chamber. Put a white piece of paper between the frame and 12:00 chamber and look to see if the chambers are lining up with the bore. Look to see that the crack of light in the barrel/cylinder gap is even from top to bottom when looked at from the side.
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