Over the years I have had "multiple discharges" the last one from a nipples that were too long and the cap next to the barrel fired when cap recoiled against the frame and hit a target next to the one the I hit with the barrel launched bullet.
Previous ones, along time ago, were from under size balls cast with brass mold that came with the cased gun.
There is nothing terrifying about having it happen, extra smoke and mess of powder streaks. The balls go down range and no one should be standing in front of you!!!