I believe I had put up two pictures on that thread prior to it being deleted. Looked through my files & I know one of these was what I used, but not positive about the second one. Anyway here goes;
This first one is of the unmodified chamber.
2nd pic is of a lengthened chamber with the blacked in portions being the metal removed in the process. Upper side shows a 5° per side angle to the cone. Lower side shows the cone cut parallel with the outer wall. On this particular drawing that gave a cone length of approximately 1˝". This would, of course, vary on individual guns.
Normal taper for the chamber itself is about 005" per inch. The barrel's OD will universally have a steeper taper than this in the chamber area so any lengthening of the chamber will result in a thinner wall at the junction of the chamber with the cone. Whether or not this creates a dangerous situation depends totally on both the barrel steel & its dimensions.
It is as patently Wrong to say that all chambers can be safely lengthened as it is to Say that None can be. It simply has to be determined by a knowledgeable person on an individual basis.
PS; You can click on the thumbnail pics & enlarge them for easier viewing.