Long before Jeb even began talking about creating fireworks in vanilla, our server had fireworks in game using this simple redstone device. I wouldn't recommending putting less than 5 TNT in the primer side, and if you max out the device the fireworks go off in space, and may very well not be visible. If you want them to go off at a specific height, I suggest placing cobwebs over the barrel at that height, it will catch TNT midflight and hold it in place.