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There is an oft-repeated account of 12,000 muskets recovered from the Gettysburg battlefield with 2 or more charges rammed down the muzzle, including one with more than 20. They all appear to trace to one source-- Major T.S. Laidley, "Breech-loading Musket," United States Service Magazine 5 (January 1865): 67-70. I don't know how "reliable" this source is, or if there are any other corroborating accounts of this fantastic number.
Perhaps Earl Hess discusses the issue in his book on Rifle Tactics?
I have read reports of multiple-loaded rifles including some reports of captured rifles loaded with cartridges all the way up to the muzzle.
I always thought such cases came from men who were psychologically unable to pull the trigger. No one noticed they didn't fire during volley fire and they simply reloaded along with everyone else.
Tim, I know I've read in a couple of places the instances of green, poorly trained troops who in their battlefield stress rammed multiple charges into their guns or fired their ramrods, but off the top of my head I can't remember the specific citations. However, Joseph Glatthaar has a good chapter in General Lee's Army ("Arms and Ammunition"), which discusses the ongoing Confederate issues of mismatched weapons and ordnance, defective ammunition, antiquated weapons, etc.
Hello H-CivWar, a student has asked if there is any reliable information on claims that a reliable percentage of panicky or confused Civil War soldiers, based on evidence of broken or damaged rifles recovered after battles, misloaded their rifles (muzzle or breech loaded), causing misfirings, explosions, rifle damage, etc. I recall some findings (as in Paddy Griffith's Battle Tactics of the Civil War) that argued that very often soldiers rammed multiple cartridges down muzzle loaded rifles, jamming them.