Had problems with a Bushmaster upper and barrel that I had assembled a few months ago. The original problem was at least 20 clicks of left windage on the rear sight to bring the bullet impact back to the left to get the rifle zeroed. Tried the non-mar mallet and block of wood approach to adjusting the FSB by tapping it from left to right.
So today I finally made it to the range and it worked pretty well. The rear sight is adjusted 1 or 2 clicks just off the center rear sight mark. Am VERY pleased with the results compared to the amount of adjustment i had to make previously.
Thanks to all for info and advice on this problem.