Just my $0.02, but it looks like the shotgun chamber and the rear end of the barrel blank were bored/turned to mate together, then soldered tight.
It's the tried/true "monoblock" type construction used elsewhere for many years.
The outside of the donor shotgun barrel looks like it was left full size to about the axle of the hinge pin, then pleasingly tapered down to meet the barrel diameter there.
Other than my wanting to make the forend wood shorter, more proportional to the barrel length - I LIKE IT ! ! !
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