I silver soldered lots of front sight ramps and recoil lugs when I worked in the Browning Arms gunsmithing shop, I can assure you that it is not necessary to heat the sight till it is red hot. The solder will be soft enough to remove well before reaching red heat, you will need to protect the bore with an anti-scale compound. An oxy-acetylene torch is the tool for the job, pay attention to the sight while heating and you'll be able to see when the solder melts enough for the sight to be removed. I have also rust blued many rifles and never felt the need to remove the sights.
