John,
HiForce 44 is 5/95 silver/tin solder. It's harder to work with but will give you higher strength if you believe that's necessary for your application. Some guns have high silver or bronze/silver brazing of the ribs. I had a Uggie that was brazed. I worked on a set of barrels from a Belgian gun that were brazed likely with a silver/bronze or silver/tin alloy.
Again, I'd be worried about using oyx/acet in that manner. I think the risks of damage outweigh the benefits. An electric heating element would be my choice if internal heating was what I wanted. Much easier to control IMO.
I believe virtually millions of guns have been put together heating from the outside, probably most or all of the Purdey's as well. H&H heats from the outside in a video I viewed.
