I had 4 chain fires.(probably a better term would be multiple discharges as only the intend chamber and the next chamber in order fired).
All of these occurrances were early in my cap & ball career and all occurred with less than perfect balls cast out of a poor(too hard) alloy.
Since, I have used only dead soft alloy and throw out the less than perfect balls.
In each case the second chamber to fire had an unfired cap securely on the nipple.