A common problem with replica Colt cap & ball revolvers is their tendency for the hammer to pull spent caps off of the nipples and to drop them into the action, causing a cap jam. I call this condition "Cap Sucking."
There have been a number of modifications developed by modern shooters to prevent this situation. Most of them involve drilling holes and installing pins, welding on cap guards, or installing expensive, after-market nipples.
I think I have found a simpler solution that just involves a little de-burring and stoning.