I agree with Clyde here - there is overwhelming evidence to support the theory that 'chain-fires' start from the nipple end of the cylinder, not the front.
Think about it you have just swaged a ball/bullet into a chamber in a way that is so tight-fitting that it actually removed all the lead that did not fit. Show me, please, how a flame can get round that, and 'back' into any adjacent chambers.
1. Make sure that the percussion caps are a tight fit on ALL the nipples.
2. Do it again.
Ditto to Clyde and Tac.