Here below is the new progress on the story:
April 3 -
Picked up the refurbished P226 from the gun store. Both sights were adjusted to centre. The gun looked and felt wonderful. It looked almost brand new, except a few scratch marks on the barrel which I knew was normal and minimal.
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April 7 -
Took the gun to the range. Shots were all to the left side, and became wider and wider until all shots flew. I looked down at the gun, and saw that the rear sight had moved to the left edge of the slide, and was just "hanging in" there. I moved it back to centre with my finger, and realized it was completely LOOSE.
My wife gave it another try, and the rear sight dropped from the slide and it took us some time to find it on the floor.
Later, when I cleaned the gun, I found that the front sight was loose, too.
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April 9 -
I hesitated when I was about to take the gun back to the gun store for warranty fix. I believed they would just Locktite the sights, which would come loose again sooner or later. I was looking for a better fix (firm but adjustable), which should be done by a gunsmith.
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April 10 -
I took the gun to Joe Dlask. He had some 2-year-old P226 night sights that were slightly bigger than the original sights, and he would install them onto my gun at the charge of $65, parts and labor all included. These night sights do not glow in the dark any more, but I believed that $65 was a good deal, and so I took it.
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April 11 -
New sights were installed and sighted in. They are tight, but will be adjustable if necessary. They are taller than the original sights, too. I think this is the best solution to the loose sight problem on the P226.