I do not use acid flux myself, just rosin flux and keep things cool enough not to burn it off. It takes a bit of practice and a good soldering iron. Once you get the hang of what the proper temperature is it becomes pretty straight forward. I rarely use a torch except to heat the breech area enough so the solder that is melted by the iron will bond to the barrels. You may seek out the book/video produced by the ACGG called "specialized gunsmithing techniques of William Nittler". He has some fixtures in the book the help with the twisting of ribs.
Steve