John,
I don't understand why you want internal heating.
As to solder, 5% silver will raise the melting temp significantly. Hi Force 44 is a 5% silver 95% tin solder and available at Brownells. The melting temp is 475F. I've used it. I see little reason to use it again.
Solder pastes with either acid or rosin flux mixed in are available. I have some I bought. Not worth the trouble and expense IMO.
My technique is to clean all the solder joint surfaces with abrasives, tin them with 60/40 tin/lead solder (360F) using acid flux, clean all the parts with soap and water with Scotchbrite pads, use rosin flux to join the tinned components. I use a large propane torch. Dewey made a fixture and burner arrangement. Heat transfer in steel is pretty quick, internal heating isn't necessary to solder barrels. Oxy-acet inside barrels would cause me concern since the mixture can get well beyond the melting temp of steel.
A few years ago, I documented how I did a re-lay, Dewey had something similar and Drew Hause compiled and posted it here.
http://www.picturetrail.com/sfx/album/view/18691676
