Solder alone will hold lugs in place, but this isn't a traditional method. On most originals you will find very shallow dovetail slots with the edges raised with a chisel. This raised material increases the holding power. I've cut slots as shallow as .015" with raised edges that held fine. You will see this same process on very thin smooth barrels but they also used braze in this case. With this process, the lugs were attached during the barrel production, so ill effects of heat from brazing could be dealt with.