Holy Macrel Red...
... now THAT was a bad day!!!
I've never had a "chain fire" but if you're using grease/lubed wads, the chances are... it was caused at the nipple end of the cylinder. Sounds like you're having nipple/cap issues.
Could be bad caps. Could be caps don't fit tight or even too tight.
If you're shooting an Italian replica, I have seen them come with standard thread, yet metric nipple/cap size. If this is the case, you won't get a 10 or 11 to fit properly no matter what you do.
If this was me, I'd make sure the nipples/caps were correct and fit perfectly, or replace them.
... just my two cents.
Tomorrow is bound to be a better day!