I wonder if the “caps” were the rolls of paper caps from a cap gun? Back in the 60’s when I was a kid we used to take a needle and stick it through the center of the little “dots” on a roll of caps. You stick it through one and then fold the strip like an accordian, sticking the needle through each cap successively. When you’ve done the entire roll you pull the needle out and what you have is a roll of caps where every single dot is touching each other and the gunpowder is mixed between them because of the needle-hole.
Then you find a way to bind it, such as with tape or something. Stick in a fuse and have fun. Did I mention we loved to blow stuff up when we were kids?
I used to put roll caps into a clear plastic pill bottle, cap it, and set them off with a magnifying glass in the sun.
Although those are still around (as far as I know), by the late '80s they had become obsolete to a new generation of plastic cups with a pretty nice charge in them. At Collector's Armory you can buy very realistic looking "blank firing" guns which are actually high tech cap guns. The caps are contained in brass cartridges that will actually activate the slide mechanism, eject the "cartridge" and reload, just like an actual semi-auto.