Mike - years ago, I straighted a friends M&P sight (like you described).
I put the barrel in a vice between two pine jaws with sight standing straight up. I then took a large pair of vice grips with padded jaws and clamped tight on the sight.
I was able to gently move the sight to true north without harming the blueing or cracking the sight/barrel.
I did not want to use heat or tonk as I thought it might do more damage.
NOTE: this was a friend's
CHEAP M&P with little to no value.
BE extremely careful if you try this on your revolver.
I AM NOT A GUNSMITH