Excellent chainfire article!
I'm repeating part here, so it's around for the next noobie search. This is from Svartkrutt.net:
1. You will probably not experience a multiple discharge:
* if the ball is of correct size and is perfect without damages or wrinkles.
* if the caps are tight and don't get lost while shooting.
* if you use correct size nipples.
* if you seat the caps on the nipples.
* if the nipples are securely tightened to the cylinder.
2. You are in danger of having a multiple discharge:
* if the balls you are using are too small or are damaged/wrinkled.
* if you have a heavily pitted chamber.
* if you keep losing caps while shooting.
* if you have too long nipples, perhaps in combination with either:
o a. caps that are not seated on the nipples or
o b. nipples that are not properly fastened (it makes them longer, same as a).
* if you ignore the other points and depend on greased chambers to prevent any chainfire. If you do this you are in danger of being nominated to the Darwin Award."
BTW, I checked around my range and no one remembers a chain fire (which doesn't mean it didn't happen and no one but the shooter noticed).
The worst cap & ball mishap I heard about was unrelated to chainfires. A shooter was trying to get a live, but stuck, cap off a nipple in the 3 o'clock position. He was using a pocket knife to pry and the chamber went off. A large piece of ball somehow ricochetted off the concrete shooting table and back into his upper arm. A few years of therapy and the shooter, I was told, has regained most of the arm's mobility.