Not relevant to the "in proof?" question, but the forcing cone wall thickness of every U.S. maker vintage double that I have measured, a few with lengthened cones (of 1 1/2" or less) was equal or greater to the wall thickness at the end of the chamber.
The angle of the forcing cone is greater than that of the exterior taper of the barrel.HOWEVER, inexpert or excessive lengthening most certainly can be dangerous; a SAC with a lengthened chamber and cones with a wall thickness of .087" at the (new) end of the chamber, with a bulge and crack
Bore view with 4 circumferential "dents" and linear tool marks from whatever apparatus was being used during reaming
Chamber lengthening is an entirely different matter, and the bulge in this Smith 16g barrel is related to a thin wall from lengthening the chambers to 2 7/8". Wall thickness was .096"
Choose your barrel guy carefully, and read the seller's "return policy" with discernment. 
