Hi,
Typically the detached pan was dovetailed into the barrel. A horizontal slot is cut in the barrel from the breech end forward. That slot is undercut to form the female dovetail. A male dovetail is filed on the inner edge of the pan such that it slides into the slot on the barrel from the breech. It should be a tight fit such that it needs to be tapped into place. Some months ago, our esteemed friend from Poland, Raszpla, shared excellent photos of this process on this board. I cannot locate that thread using the search function because I cannot find the right keywords to use. Alternatively, some pans may have been brazed on the barrel. Low-temp silver solder may also work.
dave
