I have found, with a capper, a small piece of steel from tapping can cause real hangfires.
I got a CVA rifle with a stuck ball, and full load. I got the ball out and fired out the Pyrodex. On removing the nipple, I found fouling. I cleaned and cleaned. The rifle had that load for over 20yrs and the drum and chamber was fouled. On cleaning that, I found the drum and nipple was clogged. The threads on entry to the barrel had a burr. That had fur from cleaning patches, and powder residue that was pushed against it. It resulted in a flake that would go into the opening, or gave a fairly clear path to the powder.
On that rifle, I removed the cleanout screw, and replaced it with an allen head metric machine screw. I had ground the threads to be smooth with no obstruction, the nipple threads, I chased with a tap, I then cleaned out the channel from the nipple to the barrel.
After that, no hangfires to this day. I have since done a restocking with help from a member and my wife loves it. It had a two piece stock, now an one piece stock, with brass furniture. The barrel has at least 20yr old browning on it.
That piece of fowling had all along, been the obstruction. Just a burr on the threads and it made flake in the channel.
Before the modifications, it would fire slowly. A big difference in ignition after, using Pyrodex. I use #11 nipples.
I also made the cleanout screw to stop just behind the nipple opening. I dressed the edges so they were polished smooth. No ridges. The cap flame has no where to go but into the charge with no delay. No place for the ash to be pushed into by the pressure of combustion.