The main reason for chainfires is either user error usually from the cap or nipple end. Or an incorrect size ball from the chamber end... Anyone who expiriances chain fires from the ball end regularly most likely has a cylinder with out of round holes, or a varied dimension from the end of the cylinder near to the forcing cone to a greater dimension deeper in the chamber...I have seen and measured these instances on various Revs ... Still one may think it's commin' from the flame & gases at the Ball end, and it ain't.
The most well known cause is a bad fittin' cap, bad nipples, or just a fluke...but from the rear of the cylinder is the most true statement of cause. They don't necessarily have to be missin' caps...an ill fitted cap or a nick in the nipple with all caps on will direct gases or flame to the chainfired chamber from the nipple end.
I too agree with Elmer Keith ... cap end if you have the ball in correctly and measured yur chambers with the proper ball mic or three point inside mic to be sure they are round...most cylinders will vary a few 10 thousands of an inch + or -...