RE: Sig Spacewalker II 1/4 scale
In both cases it was not the solder joint failing, but the coupler broke where the threads start, basically breaking the coupler in two. The couplers were the brass style, not sure of the manufacture, possibly Dubro or Sullivan.
I prefer the Stay-Brite silver solder available at most hobby vendors. I polish the rod a bit with the Dremel, clean everything with alcohol, apply flux, heat the joint and allow the solder to wick into the joint.