Chain fires are more often caused by poor fitting caps. The Colt Co wrote information on this and I have read it in other period literature. The cap needs to fully seat on the nipple and it should not be squeezed as this makes them oval shaped and allows hot gases to get past the cap. If you have a out of round cylinder or ball you may experience a chain fire from the front of the cylinder. If you shave a complete lead ring when seating the round ball you have sealed the chamber, if it is round.