Keith, are you turning on a mandrel? If so, something must be wonky with the bushings. The brass tube must have been off-centre when you turned it down. Can't think of anything else that would cause it. Just spitballing, but is it possible that you didn't seat the bushing inside the tube on one end? Or both ends? You wouldn't have that problem with slimlines because the tube centres directly on the mandrel. Hope this helps,
David