Steven: to address your question regarding safety of chamber lengthening; it's very likely to be safe.
I had Briley lengthen a 20g Parker with chambers shorter than 2 1/2" without problems. The safety of doing so is entirely related to the end of chamber wall thickness, and most U.S. doubles had generous walls.
That does NOT apply to lightweight British or Continental small bore game guns.
The lengthening must however be done by an expert.
An example of "inexpert" lengthening resulting in a bulge just past the end of the chamber
