Any and all comments are welcome.
Besides the spark from caps, you are also getting a larger amount of "blow-back" from your main charge. Unless I missed something, you did not list what propellant you are using. The corrosive residue is greater when you use BP or Pyrodex. When and if you pop a cap on an empty rifle, you will notice that you cap is sill in tact and in place. While firing, you will notice that they will be split wide open and sometimes, hanging in your hammer cup. Again, that is how strong the blow-back can be. That is also why some folks use flash-cups on nipple and drum assemblies. They direct this blow-back away from the stock barrel and drum. ....
Be Safe !!!