Hi Gentlemen,
I'm no expert either, but i see it close to Fearn's impression.
I would further point out that, in my view:
The black powder cylinders or, better saying, the percussion (cap and ball) cylinders, are not opened in their back; you pour your gunpowder into them like you pour it in a vase, later locking it with the bullet. The only passage to the exterior is inded the ignition cap nipple.
It happens that, when you strike the cap with the pistol's hammer to ignite the powder charge inside the chamber, plenty flames come out the nipple, as the cap is not completely tight. Therefore and potentialy these escaping flames end out igniting the nipple next door, due to their close vicinity. You would then, in a hazardous context, ignite all chambers ... either in a chain succession, which is more likely and, in such case, only shooting the whole thing without bursting in your face, or in an absolute simultaneous deflagration, in such case blowing yourself up, instead of shooting frontwards.
I guess the efect quoted in the first place is like when you want to extract rock from a stone pit. As you don't want your rocks to pulverize with the explosion, you use detonators with a (micro) time delay among them, so that the apparent simultanity achieved is 'false', with your rocks bracking apart but not becoming gravel; how's this for a comparison ?
If indeed things happened as sugested, the coming of the complete sealed cartridge has put an end to these misfire situations; in this case ignition occurs inside the ammunition ... no flames hanging around.
Pardon me for all i said, in case it's all 'bs'.
Fernando