I had two or three multi-ignitions in the early 1970s with the same revolver: a cheap, Italian made, brass-framed copy of the 1851 Colt in .44 caliber (I know, Colt didn't make the Navy in .44 caliber, dunno what I was thinking when I got it).
Anyway, my memory is hazy on it now but I do recall that the proper (12 o'clock) chamber went off, as well as the 6 o'clock barrel. The ball wedged itself against the rammer but I pried it free with a knife point.
It was a shock when it happened, as the recoil and blast were noticeably stiffer.
The next time, the 2 o'clock and 6 o'clock chambers fired with the appropriate 12 o'clock. Never found both offending balls. Boy, that was some blast and recoil when it went off!
The third time, the 6 o'clock chamber went off and wedged the ball into the rammer. This time, it swelled the end of the rammer and warped the assembly around it.
I gave up on the revolver after that and gave it to a gunsmith friend for parts.
In all three instances, axle grease or Crisco (I was prone to use either at the time) were smeared liberally over the ball. The ball was a tight fit too, as I was casting .454 inch balls for it and my Ruger .45 Long Colt.
I have never believed that flame enters from the front of the cylinder, gets past the grease and ball, and ignites the powder.
Instead, I firmly believe that the flame enters around the caps and ignites the powder.
In the 1970s, I didn't even know that the smaller No. 10 caps existed and used nothing but No. 11 caps. I had to pinch them together to make them stay on the nipple
Since those days, I've made it a point to use tight-fitting caps on my nipples. Some revolvers require No. 10 and others require No. 11. In either case the caps are pinched into an oval shape to stay on the nipples.
After capping, I use a short length of wooden dowel to carefully seat the cap firmly onto the nipple.
For the past 20 years I've rarely use lubricant over the ball. Instead, I soak a stiff felt wad in lubricant and seat that on the powder firmly before seating the ball.
I believe that seating the wad against the powder flattens the wad a bit, pushing it against the sides of the chamber and preventing flashby from the front (if, indeed, this is the cause but I remain doubtful).
The only exception to my use of wads is with conicals, whose grooves are filled with lubricant. I have some original-pattern conicals with no grooves, so I smear lubricant over these.
Since the early 1970s I've sent thousands of lead balls and conicals down range and not had ONE multiple discharge. This, from a variety of revolvers and calibers.
I don't understand how a chamber can be cut with a bottle shape, considering the drill bit enters from the front. Even if it doesn't enter straight, and wobbles in an arc, wouldn't it just create a cone-shaped chamber instead of one wide at the front, narrow in the middle and wide at the rear?
How could this be done? I'm not a machinist so I don't understand.
Now, if the bit slipped sideways a bit, I can see how you'd get an elongated chamber. A ball rammed into such an elongated chamber would allow flame to get past the ball. But I'm inclined to believe that the grease over the ball would hinder the flame quite a bit.
The mechanics of a multiple discharge may never be understood because it's not a common event. Many shooters have, "a friend of a cousin whose boss knew a guy that ..." but cap and ball shooters who have actually experienced the event are rather rare.
In 30-plus years, I've met one --- and he credited it to his own ignorance: he didn't place grease over the ball and used ill-fitting caps that frequently fell off the nipples with each shot.
In short:
Use a tight-fitting ball of .320, .380, .454 or .457 inch.
Use proper caps that fit snugly, and firmly down on the nipple.
Pinch the cap into an elliptical shape so it clings to the nipple better.
Ensure the cap is fully down on the nipple by carefully pushing against it lightly with a wooden dowel.
Seat a well-lubricated felt wad firmly on the powder, before seating the ball.
Turn to Paterson, New Jersey and sing the praises of Samuel Colt.
Hey, it couldn't hurt!