Hi Fearn,
Thanks for having dug such an amazing story
It is of common knowledge that Mr. Twain was a man of great resources; but it is obvious that he wasn't familiar with the weaponry universe. What his friend had was a 'mule killer', rather than a decent 'pepper-box'

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Perhaps Allen had better restraining his abilities to single shot pistols, while others developed more secure multiple shooters

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Speaking seriously, one can read that, the potential simultaneous shooting of all chambers was indeed more dangerous with the next generation single barrelers. Quoting for example 'answers.com':
Several models were dangerous because firing one powder charge could ignite the others, all at the same time, when proper care was not taken. This would be less dangerous than when the same thing happened in a single-barreled revolver, because in the pepperbox at least all the bullets could freely exit the muzzle. This was perhaps the main reason for the pepperbox's survival after more modern revolvers came along.
Just for perusal, i attach here a Net picture of what could have been George Bemis's Allen 'pepper-box', as well as a set of photos of a single barrel 'muff pistol' made by the same author, from my own little collection. Worthy of note is the fact that both these two examples have a front loading percussion system, whereas the pepper box i've opened the thread with, belongs to a later generation; certainly a much safer system. There are no powder charges, but metalic (pinfire) cartridges.
Fernando
BTW, pepper-boxes are not aimable guns; you just shoot them (hip level) at close range ... impossible to miss the mule

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