Sounds like the nut was not properly staked to the screw and it may have tightened up when you adjusted the sight, thus "freezing" the screw in the sight body. You can make a little spanner to turn the nut by taking a small screwdriver and filing away the tip of the blade corresponding to the area that would contact the screw. You're basically making a "spanner wrench" out of a screwdriver. You don't want the nut tight when you re-stake it. I usually leave about 1/8-turn or so of clearance. You want just enough clearance that everything moves freely, but not enough that the blade is able to wander around.
Once you get the nut loosened up and repositioned, you can easily re-stake it on the screw using a center-punch. I usually regrind a punch to 60-degrees included angle. You can support the other side of the screw (the head side) on a lead bullet, if you don't have a small block of lead handy. It doesn't take much to stake the screw, so don't whack too hard. Two or three light taps will usually do it.
Naturally, you want to remove the sight from the gun when you are monkeying with it.
Once you get the nut loosened up and repositioned, you can easily re-stake it on the screw using a center-punch. I usually regrind a punch to 60-degrees included angle. You can support the other side of the screw (the head side) on a lead bullet, if you don't have a small block of lead handy. It doesn't take much to stake the screw, so don't whack too hard. Two or three light taps will usually do it.
Naturally, you want to remove the sight from the gun when you are monkeying with it.
