Alex makes some excellent points above that I'd like to expand upon a bit. Solder, whether soft or hard, flows towards the heat. The proper way to solder, and many never learn this art, is NOT to stick the solder into the flame. You continue to heat the parts until when you touch the solder to the parts, out of the flame, the solder melts and flows, by capillary action, into all of the crevices in the joint, to its very core. This holds true whether you're soldering half-inch copper pipe with 50/50, or whether you're building a double rifle with silvaloy! What this means is that if you expect the solder to flow right to the centre of a shoelump, or a chopper lump, or whatever, then the centre of that assembly MUST BE at least as hot as the exterior.

While it is not impossible, it is EXTREMELY difficult to accomplish this with a hand-held oxy-acetylene torch. What is even more difficult is to achieve that interior heat without bringing the exterior to a temperature at which the properties of the steel are negatively affected and degraded. That's why Alex uses a brazing furnace! He can set a given, safe temperature, at which he knows his solder alloy will flow, introduce the parts to be joined, and wait and watch until those parts become heated through, uniformly, and the silver solder flows into the joints. There are men around who have the eye and the experience to do this with a torch, but they are few and far between, and their skill borders on whizardry! Please, don't ask the guy down at the corner, who builds utility trailers for a living, to do this job for you.

Now I'll get off my soap box!