Heating the sight to red heat will likely be enough to melt the braze (or solder) metal, but you need to protect the bore from scaling. If you don't have a commercial anti-scale compound, you can make a tight fitting wood dowel about 6" long to drive into the bore under the sights. This will char and take up all the oxygen in the little bit of air space between the dowel and the bore thus preventing scale formation. The localized heating will not harm the barrel metallurgically.
RAN
