Wikipedia
A cap gun, cap pistol, or cap rifle is a toy gun that creates a loud sound simulating a gunshot and a puff of smoke when a small percussion cap is exploded. Cap guns were originally made of cast iron, but after World War II were made of zinc alloy, and most newer models are made of plastic.
Cap guns get their name from the small discs of shock-sensitive explosive compounds (roughly in diameter) that provide the noise and smoke, effectively the same as the Maynard tape primer and percussion caps used in real firearms of the mid to late 1800s but usually smaller and made from cheap plastic or paper. Some are arranged in plastic rings of eight or twelve. There are also single caps, roll caps (of 50 to 500), and disk caps, all of which are actually extremely small versions of percussion fireworks. Armstrong's mixture is often used today as the explosive, but previously the tiny powder charge was a simple mixture of potassium perchlorate, sulfur, and antimony sulfide sandwiched between two paper layers which hold in the gases long enough to give a sound report when the cap is struck.
Usage examples of "cap gun".
Dad said it broke open at the top like a kid's cap gun and wouldn't shoot worth a damn, but it was kinda comfortable to have it along.
The badge looked like the sort of thing that had come in a plastic bag with a cap gun and whistle.
I pointed at him and the little automatic snapped a little louder than a cap gun, and he spun and yanked the trigger of the rifle while some spectator in the back of my mind peered at him and told me that the fool had forgotten to work the bolt action.
This could very possibly be the end of it, a long-odds chance of a mortal wound at the hand of a jealous husband wielding something just a little bit better than a cap gun.
Stephanie's juggling lace hankies and firing her cap gun at the ceiling.