Silver solder terminology is often a bit confusing. Silver brazing is what a lot of people call silver solder. I do not usually recommend a silver braze joint on a rifled barrel, but often put tactical shotgun front sights on using it. This is a compound of silver and copper, which, depending on the exact alloy, melts from approximately 1200 to 1350 degrees F. It requires high temperature flux, usually borax based. The joint is usually fitted up, fluxed and then the solder is put in by capillary action. The solder will follow the heat into the joint and make a very strong bond. The problem with silver braze is that the heat necessary will alter the heat treat of the metal, and also will scale raw steel if not protected with anti scale paste. The bluing will be affected, but sometimes a good enough job can be done that touch up blue will make things look OK on a "using" gun.
The manufactuers use silver braze when putting on sight bases, bolt handles and joining shotgun barrels to monoblocs, but they normally do this in an atmosphere furnace to avoid scale, and they refinish the part after brazing.
Silver solder is a mix of silver and tin, the ratio usually asbout 5% silver to 95% tin. This is about 4 times as strong as regular tin/lead solder, and I have used it to mount front sling swivel bases on exensive rifles. It will hold if the joint is properly done. You can mount sights using either the "sweat" system" or the capillary joint type assembly. The sweat system is best if you don't wish to reblue the gun, but it is a lot of trouble and takes some skill. I find silver brazing, as a process, easier. If you wish to sweat things together you tin each part by fluxing and heating the surface and then apply solder to form a microscopicly thin coat. A flux brush or small wire brush is needed to remove excess solder (while hot). Then the parts are assembled with a bit of flux between them and heated until they stick together. This process is 450 - 550 degrees F. or so, depending on the blend of alloy. This won't damage barrel heat treat and if only minor amounts of flux are used, the bluing won't be damaged. I find heat control is critical in making a good joint and is not a skill that is developed without some practice. The flux will burn easily and make it so the solder won't flow.
There are silver braze pastes and silver solder pastes which incorporate their respective fluxs in a mix with ground up bits of braze and solder. These are viable in certain situations, but again, some experience is needed.
To address your problem after my long winded reply, you can either silver solder or silver braze the sight base on your shotgun barrel. The silver braze will be stronger and easier to do, but may mar the bluing and internal bore surface (unless you use anti scale inside the barrel). The silver solder will probably be OK, but the joint will also need to be fitted very close to the barrel (.003 clearance or less) in order to have strength. You might want to practice on some scrap before working on your gun.
Good luck,
Scott
