At this point in time I will offer no opinions as to what solder is suitable and what solder is not. However, I will point out that comparing tensile strengths of various solders is totally irrelevant as the solder is not placed in tension. The question is one of shear strength. The shear strength of the solder is the property that should be considered. The area of the joint is quite large so given a good quality joint the required shear strength of the solder may be less than intuition would lead one to think. I would hazard to guess that the quality of the joint may be of more importance than the choice of solder.

Also the stress is related to backthrust of the cartridge rather than pressure. I have noticed a tendency for people to think solely in terms of pressure when considering the stresses placed on a fire arm, to the exclusion of other stress factors which are often more important. The prime example being the cycling operation of a recoil operated self loading handgun where the cartridge impulse is far more important than the pressure which is almost irrelevant.