Longknife,
What has been said above about HiForce solder from Brownell's is correct. I have used it for years with great success, including the building of eight side-by-side double rifles. What needs to be added is that the best solder in the world is useless without proper technique and proper soldering fluxes. Brownell's sell the correct solder to pre-tin all of your parts. This is an acidic flux, which is necessary to get the proper "wetting" of steel alloys. You then need to remove all traces of that acid flux by washing the parts well with hot water and detergent. If you don't do this you will encounter serious rusting problems down the road. Final assembly of the pre-tinned parts should be done using a liquid rosin flux. You can find this at most Radio Shack stores. The old-timers used powdered rosin. It's difficult to find and the liquid is much easier to use. Just paint it on the surfaces to be joined, let it dry for half an hour, clamp the parts together, and apply heat and more Hi-Force. I buy cheap artists' brushes at the local hardware store to apply the rosin flux with.

PS: Don't be alarmed if the rosin flux catches fire when hit with the torch. That is perfectly normal. Also, it leaves a wicked sticky mess on hands, brushes, and the final assembly. Gasoline dissolves the residue easily, a job best done outdoors for safety's sake!

PPS: One caveat!! Despite the fact that Brownell's advertise that parts joined with Hi-Force solder may be hot-blued, I have personally witnessed such joints damaged when left too long in a hot bath. Take care in that regard!