A silver bearing solder needing 650degrees is not really a high temp silver solder. I cast bullets at 850 degrees - still not high heat as the high temp silver solders/brazing temps go.

On the other hand, bright red heats are considered high temp silver solders, which is in the 1,200F to 1,300F range.

On a good silver solder, a 1 mil bond can have up to 75,000PSi shear strength - much more than needed for joining two barrels.

Soft, ie: low temp solders are all that is needed, I am sure. Contacting Colin Stolz of this site should give you some answers.

None of the original ML SXS's or CF rifles both up to 4 bore, were joined with high temp solders.

Good luck on the project. I've wanted a 14 bore SXS for decades, since 1980's, reading Forsyth's book, which is why I had a "cheap single" (his words) made up for my use.

A number of guys here have built and regulated doubles.

I'm sure someone will suggest a certain book for you. SRY I cannot remember the author's name. It will probably come to me at 3AM.