the hammer would indeed hold down the nipple, but could not prevent a more catastrophic failure.
I'll see if i have saved any original sources for this. i did find an on line article from the American Society of Arms Collectors about these which contained this about the Belgian cone in barrel method, which the article calls the second method:
The first and second
methods of alteration were used primarily during the
1840's and early '50's. With the advent of the idea of
rifling the barrel in these muskets and applying long
range sites, the first and second methods became unserviceable
due to the increased level of gas pressure
and also due to the position of the cone on the barrel.
These problems lead to the wide use of the third
method or rebottomed barrels.
I think that the cones were put into a bump created on top of the barrel created by some sort of device inserted into the barrel which pushed up the metal to create a bump which then was drilled. I suppose that might cause some structural weakness there, especially if there was some internal flaw or stress already existing there.
I suspect that this system was usable, so long as the bore was not rifled, and installation and maintenance done correctly. As always, the chance of a failure that could be fatal means that even a bit of a chance is something to be avoided!
I am also certain that soldiers at the time would have complained bitterly of the danger and unsuitability of these older, large bore weapons in the hopes that they could exchange them for "modern" .58 caliber rifle-muskets, so we can't always believe what we read in original sources.