Sir:
Forcing cones are a good thing. As long as the creation/elongation of them does not thin the barrel walls too much ahead of the chamber. This is the primary danger in old doubles because this is where pressures are the highest.
To determine wall thickness, you need a wall thickness gauge. A good gunsmith will have one. And will be able to advise you on safety.
The function of the cone is to "funnel" the shot and wadding into the bore as they leave the cartridge. The presence of a cone does not raise pressure. It lowers it. What generally loosens an old double is hot, heavy-charge modern cartridges. Americans love them. Old doubles don't.
Read some of the McIntosh "Shotguns and Shooting" books for more.
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