For rejointing barrels I have only ever used tin with a dash of silver, maybe 5%, as per traditional "tinning" of sights, barrels etc.
Brownells used to sell it by the pound under the name Hi-force 44. Flows at 475 F and is rated at 14,000 to 28,000 psi tensile strength.

Just thought I'd toss this in as tin was traditionally used and it seems to be left out of the mix in this thread.

My knowledge of modern epoxies is obviously "dated" to say the least! I had assumed all epoxies softened with heat. That's how we used to get them apart!