Most cap and ball revolvers have the nipple set into a mortise, and the hammer is configured to trap most particles. Of course, we know some get out, so we always wear eye protection.
In a reenactment situation, I suppose shooting glasses aren't too authentic, so they are not used.
I would try to locate some Remington caps, and see if they hold together better. If not, cut down on your powder charge, which would decrease the amount of gasses exiting the flash hole in the nipple.
If all else fails, maybe the flash holes have eroded, and new nipples are in order.