As usual, I am coming in late in a conversation ..... I have used soft solder on numbers of firearms over the years. For example, I have cut off a worn front sight on a broomhandle Mauser and soldered a newly fabricated one on the flat that remained. Many of my blackpowder handguns have had newly fabricated sights soldered in place (since they never shoot to point of aim out of the box.)
Some of the better soft solder alloys will have tensile strengths in the 4000 psi. range. Brownells and Midway are good places to look. You can find the same stuff (sometimes) at hardware stores and such (check the specs on the roll or box.) Fit, preparation, and the correct flux are the keys to a good job.
Remember that the surface area of the solder bond will ultimately dictate how strong the end result will be. If you are trying to solder something that only presents a tiny surface area, it may not be a good candidate for a solder repair. As an example, if you try to increase the height of a thin sight blade by soldering a thin extension to the blade, you would likely break it off the first time it gets bumped.
If you like numbers, it's real easy to calculate the theoretical "holding" strength of a solder joint. Just measure and calculate the area of the solder joint (just the one surface.) Then multiply the area (in square inches) times the tensile strength of the solder. This should give you the "pull' that the sight or other part should withstand. Probably, of more importance, is the shear strength of the joint. This would be more representative of a "sideways" knock. A good rule of thumb is to use 60 - 70% of the tensile strength. So if you get 8 ounces for the "holding" power, it isn't a good candidate for solder. Typical dimensions of the bases of most sights will give you many pounds of holding force - plenty, unless you are pulling out fence staples with your front sight!
This is probably more that you ever wanted to know, so I'll shut up.
