Here are some thoughts from one who has built 3 and is on # 4. Regular tin/lead solder will do fine but, as Tony pointed out, Force 44 tin/cadmium solder from Brownells is far superior. It has a tensile strength of about 28.000 psi versus 4-5000 psi for tin /lead solder. Silver is NOT necessary and I HIGHLY recommend against using it. Tin/lead melts at about 400 deg.F. Force-44 melts at about 600. Silver melts at 1150 to 1350. You need near "red heat" to melt silver solder and at those temperatures, molecular changes are occuring in the steel which are NOT strength-enhancing. Whether you use tin/lead or Force 44, it is necessary to pre-tin both the sockets of the mono-bloc, and the shanks of the barrels, before final assembly. IT IS IMPOSSIBLE to get 100% coverage and cohesion by relying on capillary action alone!!!! I'VE TRIED SEVERAL TIMES, AS EXPERIMENTS, THEN DISASSEMBLED TO INSPECT. YOU'RE LUCKY IF YOU ACHIEVE 60% COVERAGE THAT WAY.

There are two advantages to threading rather than soldering. Of course, you need a lathe or a machinist friend to do this. First, you are able to achieve a "crush fit" between the barrels and the monobloc which makes the seam very difficult to see if done right. It's difficult in the extreme to achieve that with a soldered joint. That's why makers like Beretta roll-engrave that seam, to camouflage it.

Secondly, once they are threaded in, you don't have to worry about applying longitudinal pressure on the barrels, later, when you're soldering up the forearm hook, the spacer-blocks, and the ribs. When you get that far, the whole assembly needs to be heated well above the temperature where a solder joint in the mono-bloc will flow.

A word to the wise, from one who had it happen to him. If you solder, rather than thread, then you must make up a spring-loaded jig to hold the barrels into the mono-block during the rib-laying process, If you clamp things up rigidly, the barrels WILL expand and lengthen during the heating process, to the point where they may actually accordion on you. A spring-loaded jig allows the barrels to expand longitudinally while still keeping them in close contact with the mono-bloc. That ain't hearsay folks, I ruined two good barrels to discover that, and wasted a hell of a lot of time and effort that I'd spent in turning and polishing them as well. Boy, am I ever long-winded today!