In this instance it happened on one of my .50 cal. Hawkens. Probably the CVA as it is rifled 1:66 and I like to shoot round ball. Port is too large and let gas escape, blowing off cap and re-cocking the hammer, as I recall, to half cock.
Not a good thing. Had a friend who's brother overloaded a T/C Hawken once. Hammer went wizzing by, narrowly missing surgically removing his right ear.
Please, please wear eye protection. TrackOfTheWolf will sell you new nipples but they don't stock eyeballs. Having had four (non-shooting-related) surgeries, I can say without qualification it ain't fun, nor cheap. I could have bought beau coups guns for what my two insurances did not cover. Play safe.