I remember those well! My favorite was my little plastic bullet firing derringer that I saved up to buy for what seemed like forever. I remember that the bullet used a cap inserted into a two piece metallic cartridge with the hollow red plastic bullet placed on top. It didn't take me long to figure out that emptying a firecracker into the hollow of the plastic bullet made a much more satisfying bang when shot (and the bullet would hurt like h*ll if it hit you).
Kids today just don't get to have clean, decent, creative fun anymore.