Assuming the gun has been evaluated by a doublegun specialist smith:
How much can one shoot a vintage double?
Depends if it is a lightweight 3 Ton proved British game gun, or a heavy 12b 3 1/2 Ton proved Waterfowl gun. What is most prone to break?
The wood at the head of the stock, or possibly the wrist.The loads used would be under 8000 psi.
Recoil is as important as pressure Vern. For what load was the gun proved, and when?Scroll down toward the bottom here for some thoughts
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit 
