Here's another explanation:
It took me a minute or ten but I finally figured it out: The percussion caps were the wrong size for the nipples on the cylinder. I had run out of #10 caps and simply squeezed a #11 cap to a snug fit in the nipples. This is practice causes no harm when I use it on my side-lock .32 squirrel rifle but it proved to be a dangerous practice for revolvers. The flash and flame that engulfed the cylinder was finding its way into the opening left by my “pinching” of the oversized caps. I proved this by loading one chamber with a charge but just capping the others. I would find one cap burned for every two or three firings. When I switched back to #10 caps on this revolver the problem was cured. Save yourself some grief. Stick to the manufacturer's recommendation for percussion cap size.