I used to have the revolver style cap guns with the red ring of caps like GC8 mentioned. During a summer holiday, me and my best pal at the time were playing with them in my papa's garage.
There was string and WD40 in the garage, so we put the caps down on the ground, made a big circle with the string, sprayed it with WD40 and then hit the caps on top of the string with a hammer.
Didn't expect it to work, but the whole circle lit up making a ring of fire. :L My Papa unfortunately noticed and came and stamped it out - probably a good thing with all the chemicals in the garage. Lol
Still, good times!