Hi,
I am not sure when folks write they use silver solder, they mean hard silver solder that must be heated well over 1100 degrees F or lower temperature "silver bearing" solders some of which can become fluid around 470 degrees. I use low temperature silver bearing solder like Stay Bright and Hi-Force 44. Mapp gas is the most efficient but the work can be done with propane torches. As others wrote, clean surfaces are a must. Some guys tin the surface of the barrel first and then place the lug on top and heat until the solder melts again. I prefer to sweat the joint. I clean the surfaces, brush on flux, and then wire or clamp the lug to the barrel. I turn the barrel in a vise on its side such that the lug is horizontal and place a piece of solder wire on the top edge of the lug's base. Then I heat the lower edge until the solder is drawn under the lug toward the heat. When I see solder show up along the lower edge of the base where I am heating, I know I have a good solid joint. On the octagon section of your barrel you can simply dovetail the lug in place.
dave