When I was a six year old I used to imagine I could convert my cap guns to shoot real bullets.
Actually a few of them can be converted after a fashion but it strikes me that it would be just as easy to make one from scratch.
My brother and I had some Nichols (sp) cap guns..When I was seven, one night at dinner, I stacked 13 of the little round caps in the shell, and fired it at my brother..I was expecting a big bang. What I got was the "bullet" hitting the bar in the front of the barrel, then going back up through the barrel, hitting the hammer, then travelling forward along the side of the gun. It barely missed my brother's head, but did make a hole in the cement block wall in our Wherry Housing abode..
I still remember some satisfaction, at my older brother's screaming, "She tried to kill me. She tried to kill me." I remember that better than the meal I am sure I missed after being sent to my room for being "stupid", and cramming 13 caps into my little derringer.