In working with the ribs on my first Dbl. rifle project I discovered there is quite a difference in soldering shotgun ribs as opposed to the much heavier rifle barrels. The amount of heat needed is considerably more to get good flow and adhesion of the solder. Then there is the more critical alignment issues with rifle barrels. In using the traditional method with wire and nails there is the added problem of soldering around the nails and not soldering them to the ribs...

I made this little fixture to hold everything at once while not crowding the work area. Since making the orginal I've since moved the four (two top and two bottom) verticle alignment barrel screws closer to center. As you can see they were virtually useless with this particular project. The open design doesn't sink any heat away from the barrel/ribs while allowing space to work around the fixture. This type fixture is featured in the second addition of Ellis Brown's book. Mine is made of 1/4 X 1 inch aluminium simply bacause I had it on hand. You could easily use steel or I think ideally, a section of 2X3 square steel tubing. The screws are stainless to avoid solder sticking to them. Originally I only made one using wire and nails farther back the rib but it worked so well I've made a second that together should be all that's needed to hold an entire assembly - barrels, top and bottom ribs.