Well, I did find a gunsmith, and he says S&W specs for the mod 19 are rather loose, 0.006 to 0.012, so mine is just within specs. He inspected the gun and confirmed that it seemed to have had very little use, so the gap probably existed from the beginning. Finally, he made clear reducing the gap would be a major job. The barrel would have to be rotated into the frame one full frame, which would probably require machining the end of the barrel to allow adequate gap, and moreover the extractor rod would now foul the shroud, so it too would have to be shortened! I think I'll try to live with it.