But it has, and does still happen. One of the more interesting things found by people going over Civil War battlefields (shortly after the battle) was the number of ramrods (on both sides), in places where they only could have gotten if fired there. And there are cases where both game and the enemy were actually hit by fired ramrods.
Bear in mind - - - huge numbers of those soldiers had little, if any, experience with firearms.
Huge numbers of immigrants got off the boat broke and walked right into Union Army.
They got very cursory instruction in how to load and fire a weapon.
Many of the weapons had multiple powder charges and balls - - they had been fired sans caps & then reloaded.
OTOH - - try to find any mention at all of the same thing happening during either the Revolutionary War or the War of 1812.
Troops on both sides there were well drilled & very familiar with the way their arms worked.
Must have been some intense heat. As tight as the lead balls fit in the chambers on my Lyman 1858 I can't see enough of anything melting down, and seeping into the charge enough to contaminate it.
Reproduction 1862 Colt Pocket pistol. It still has the remnants of two balls in the chambers. The Bore Butter I smeared on the cylinder faces melted and ran into the chambers fouling the powder charges in two of the chambers.
The nipples were impossible to remove & I ended up trying to worm the balls out. That also failed. All I ended up doing was putting a big hole in the ball.
I doused the cylinder with lighter fluid and set it on fire inside of the hollow of a cement block - something like three or four times, before the powder charge finally went off.
The balls are still in the chambers BTW.
& no - the heat was not intense at all. No hotter than a blued gun gets after being fired a few times on a Summer day.
Also - browse around ML users for a bit and take note of the huge number of them that grease the face with Crisco......Crisco turns lliquid just by looking at it.
Oops - forgot - one other thing...
According to the Log Cabin - where I bought my BP stuff back in the 1970's, greasing the faces was optional on a revolver using round balls - just as a safety measure if the balls are undersize and don't shave off a ring when rammed.
OTOH - it's required if using conical bullets.
CB's don't shave a ring of lead like a ball does. They have a slight skirt on the base that upsets and fills the chamber.
Even with a felt pad, a flashover can happen.
& yes - my good friend Tom had a chainfire with my 1862. It flashed and 5 chambers went off. Scared the crap out of him, but, he wasn't hurt.