In the rare event that a patched ball should find its way down the barrel of a front loading gun before any powder, then it is quickest to remove the nipple, add powder, and fire the obstruction out. The problem is that powder granules will foul the threads, making it easy to mess them or the nipple or both up. The powder has to go down and make a 90 degree turn to get to the barrel, causing the back up. When that screw you talk about is removed, leaving the nipple in place, it is much easier to slip in 5 grains of powder, strait into the barrel, to clear the booboo. Sometimes the priming horn has a tiny funnel which fits right past those threads leaving them free of powder residue, for screw replacing.
At least that is my discovery.
HOSS