Several years ago I shot a friends S&W 645 and I was impressed- guess that sorta dates me. So the next gun show I started looking and eventually found one and it came home with me. Every time I took it out I was discouraged -good groups, but high. That rear sight was as cranked down as a Bomar can get. So my prize gradually migrated to the back of the safe.
I happened to be reading a topic when I came across an off hand comment that someone observed that on some of their guns when placed upside down on a flat surface the line of sight was such that the axis of the bore pointed upward.
This would mean that the trajectory would never cross the line of sight. Gol-that won't work! Heavy bullet moving slowly down the barrel and recoil forces the muzzle up before the bullet leaves the barrel.
I had been following the rule "move your rear sight in the direction you want the bullet impact to move" . In this case moving the rear sight up brought the groups down. Exceptions for rules.