With ordinary tin/lead solder 356°F is the point to be concerned about. I seriously doubt you are going to still be holding on to a gun when the bbls are that hot. 356° is the eutectic point (that point at which the entire mix goes from solid to liquid at the same time) of tin/lead at a mix of 66/34% respectfully. Any other tin/lead alloy will have a higher total melting point but will still get mushy & lose strength at this temp.There are of course soft solders of other alloys than strictly tin/lead which may have higher melting points, such as some of the lead free soft solders.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra