Larry;
If a neophyte to old guns made that mistake I would tottally agree with you, he was just following oft repaeted advise which is in most cases too general & broad in its scope, with no included caveats.
I can really speak only for my self but I will tell you if I ever make the mistake Fergus did it "WILL" be from Utter Foolishness/Carelesnesss.
I truly do feel that if You did it it would be the same & that When Fergus did it was the same.
Surely he examined the bore to be sure there wasn't a Ditrt Dauber's nest or other obstruction there. A chamber end having a step or extremely steep angle is "Extremely Easy" to see visually. It should certainly have been throughly checked out & the length of the shells to be fired determined prior to ever being fired in this very old & irreplacable gun. I don't recall now exactly what gun it was, but seem to recall it was from perhaps the 1860's or no later than the 1870's.
It is very rare to encounter a chamber of this type & they will be mostly found in these Very old guns. I don't know it for a fact but I seriously doubt a modern proof house would proof such a gun without the cones being re-cut
When working with these very old guns one needs to mostly ignore all thoise general statements & treat each on an individual basis. The main fault I find with Bell's articles is that im my opinion he was just too casual & cavalier about the subject, giving virtually a blanket recomendation with no exceptions cited.
This just really Ain't the case as shown by Fergus' experience.
