When silver soldering sights on a barrel, mark the area the sight will be solder too, and clean the surface. I usually ruff it up with a dremel tool after taping the area around it to protect it. Then I remove the tape, and I heat the area and sight with a propane torch until it reaches proper temp where the silver solder liquefies, and clamp the sight till cool.
I also usually cut the proper amount of solder, flatten, and put between the sight, and barrel, clamp with a metal clamp and heat while fluxing. When the solder liquefies the sight will settle.